Add multilingual support in Hugo

Implements:
* support to render:
  * content/post/whatever.en.md to /en/2015/12/22/whatever/index.html
  * content/post/whatever.fr.md to /fr/2015/12/22/whatever/index.html
* gets enabled when `Multilingual:` is specified in config.
* support having language switchers in templates, that know
  where the translated page is (with .Page.Translations)
  (when you're on /en/about/, you can have a "Francais" link pointing to
   /fr/a-propos/)
  * all translations are in the `.Page.Translations` map, including the current one.
* easily tweak themes to support Multilingual mode
* renders in a single swift, no need for two config files.

Adds a couple of variables useful for multilingual sites

Adds documentation (content/multilingual.md)

Added language prefixing for all URL generation/permalinking see in the
code base.

Implements i18n. Leverages the great github.com/nicksnyder/go-i18n lib.. thanks Nick.
* Adds "i18n" and "T" template functions..

4 files changed, 99 insertions, 25 deletions

diff --git a/commands/benchmark.go b/commands/benchmark.go
index 3f5aa6ef3..53d2c8e9e 100644
--- a/commands/benchmark.go
+++ b/commands/benchmark.go
@@ -57,7 +57,7 @@ func benchmark(cmd *cobra.Command, args []string) error {
 			return err
 		}
 		for i := 0; i < benchmarkTimes; i++ {
-			MainSite = nil
+			MainSites = nil
 			_ = buildSite()
 		}
 		pprof.WriteHeapProfile(f)
@@ -76,7 +76,7 @@ func benchmark(cmd *cobra.Command, args []string) error {
 		pprof.StartCPUProfile(f)
 		defer pprof.StopCPUProfile()
 		for i := 0; i < benchmarkTimes; i++ {
-			MainSite = nil
+			MainSites = nil
 			_ = buildSite()
 		}
 	}
diff --git a/commands/hugo.go b/commands/hugo.go
index 7afd78a9d..57a426458 100644
--- a/commands/hugo.go
+++ b/commands/hugo.go
@@ -46,10 +46,10 @@ import (
 	"github.com/spf13/viper"
 )
 
-// MainSite represents the Hugo site to build. This variable is exported as it
+// MainSites represents the Hugo sites to build. This variable is exported as it
 // is used by at least one external library (the Hugo caddy plugin). We should
 // provide a cleaner external API, but until then, this is it.
-var MainSite *hugolib.Site
+var MainSites map[string]*hugolib.Site
 
 // Reset resets Hugo ready for a new full build. This is mainly only useful
 // for benchmark testing etc. via the CLI commands.
@@ -287,6 +287,7 @@ func loadDefaultSettings() {
 	viper.SetDefault("ArchetypeDir", "archetypes")
 	viper.SetDefault("PublishDir", "public")
 	viper.SetDefault("DataDir", "data")
+	viper.SetDefault("I18nDir", "i18n")
 	viper.SetDefault("ThemesDir", "themes")
 	viper.SetDefault("DefaultLayout", "post")
 	viper.SetDefault("BuildDrafts", false)
@@ -323,6 +324,8 @@ func loadDefaultSettings() {
 	viper.SetDefault("EnableEmoji", false)
 	viper.SetDefault("PygmentsCodeFencesGuessSyntax", false)
 	viper.SetDefault("UseModTimeAsFallback", false)
+	viper.SetDefault("Multilingual", false)
+	viper.SetDefault("DefaultContentLanguage", "en")
 }
 
 // InitializeConfig initializes a config file with sensible default configuration flags.
@@ -490,6 +493,8 @@ func InitializeConfig(subCmdVs ...*cobra.Command) error {
 			helpers.HugoReleaseVersion(), minVersion)
 	}
 
+	readMultilingualConfiguration()
+
 	return nil
 }
 
@@ -506,7 +511,7 @@ func watchConfig() {
 	viper.OnConfigChange(func(e fsnotify.Event) {
 		fmt.Println("Config file changed:", e.Name)
 		// Force a full rebuild
-		MainSite = nil
+		MainSites = nil
 		utils.CheckErr(buildSite(true))
 		if !viper.GetBool("DisableLiveReload") {
 			// Will block forever trying to write to a channel that nobody is reading if livereload isn't initialized
@@ -632,6 +637,7 @@ func copyStatic() error {
 func getDirList() []string {
 	var a []string
 	dataDir := helpers.AbsPathify(viper.GetString("DataDir"))
+	i18nDir := helpers.AbsPathify(viper.GetString("I18nDir"))
 	layoutDir := helpers.AbsPathify(viper.GetString("LayoutDir"))
 	staticDir := helpers.AbsPathify(viper.GetString("StaticDir"))
 	walker := func(path string, fi os.FileInfo, err error) error {
@@ -639,8 +645,13 @@ func getDirList() []string {
 			if path == dataDir && os.IsNotExist(err) {
 				jww.WARN.Println("Skip DataDir:", err)
 				return nil
+			}
 
+			if path == i18nDir && os.IsNotExist(err) {
+				jww.WARN.Println("Skip I18nDir:", err)
+				return nil
 			}
+
 			if path == layoutDir && os.IsNotExist(err) {
 				jww.WARN.Println("Skip LayoutDir:", err)
 				return nil
@@ -684,6 +695,7 @@ func getDirList() []string {
 
 	helpers.SymbolicWalk(hugofs.Source(), dataDir, walker)
 	helpers.SymbolicWalk(hugofs.Source(), helpers.AbsPathify(viper.GetString("ContentDir")), walker)
+	helpers.SymbolicWalk(hugofs.Source(), i18nDir, walker)
 	helpers.SymbolicWalk(hugofs.Source(), helpers.AbsPathify(viper.GetString("LayoutDir")), walker)
 	helpers.SymbolicWalk(hugofs.Source(), staticDir, walker)
 	if helpers.ThemeSet() {
@@ -695,31 +707,52 @@ func getDirList() []string {
 
 func buildSite(watching ...bool) (err error) {
 	fmt.Println("Started building site")
-	startTime := time.Now()
-	if MainSite == nil {
-		MainSite = new(hugolib.Site)
-	}
-	if len(watching) > 0 && watching[0] {
-		MainSite.RunMode.Watching = true
+	t0 := time.Now()
+
+	if MainSites == nil {
+		MainSites = make(map[string]*hugolib.Site)
 	}
-	err = MainSite.Build()
-	if err != nil {
-		return err
+
+	for _, lang := range langConfigsList {
+		t1 := time.Now()
+		mainSite, present := MainSites[lang]
+		if !present {
+			mainSite = new(hugolib.Site)
+			MainSites[lang] = mainSite
+			mainSite.SetMultilingualConfig(lang, langConfigsList, langConfigs)
+		}
+
+		if len(watching) > 0 && watching[0] {
+			mainSite.RunMode.Watching = true
+		}
+
+		if err := mainSite.Build(); err != nil {
+			return err
+		}
+
+		mainSite.Stats(lang, t1)
 	}
-	MainSite.Stats()
-	jww.FEEDBACK.Printf("in %v ms\n", int(1000*time.Since(startTime).Seconds()))
+
+	jww.FEEDBACK.Printf("total in %v ms\n", int(1000*time.Since(t0).Seconds()))
 
 	return nil
 }
 
 func rebuildSite(events []fsnotify.Event) error {
-	startTime := time.Now()
-	err := MainSite.ReBuild(events)
-	if err != nil {
-		return err
+	t0 := time.Now()
+
+	for _, lang := range langConfigsList {
+		t1 := time.Now()
+		mainSite := MainSites[lang]
+
+		if err := mainSite.ReBuild(events); err != nil {
+			return err
+		}
+
+		mainSite.Stats(lang, t1)
 	}
-	MainSite.Stats()
-	jww.FEEDBACK.Printf("in %v ms\n", int(1000*time.Since(startTime).Seconds()))
+
+	jww.FEEDBACK.Printf("total in %v ms\n", int(1000*time.Since(t0).Seconds()))
 
 	return nil
 }
diff --git a/commands/list.go b/commands/list.go
index 5267a4f8b..bc5bb557a 100644
--- a/commands/list.go
+++ b/commands/list.go
@@ -57,7 +57,7 @@ var listDraftsCmd = &cobra.Command{
 			return newSystemError("Error Processing Source Content", err)
 		}
 
-		for _, p := range site.Pages {
+		for _, p := range site.AllPages {
 			if p.IsDraft() {
 				fmt.Println(filepath.Join(p.File.Dir(), p.File.LogicalName()))
 			}
@@ -88,7 +88,7 @@ posted in the future.`,
 			return newSystemError("Error Processing Source Content", err)
 		}
 
-		for _, p := range site.Pages {
+		for _, p := range site.AllPages {
 			if p.IsFuture() {
 				fmt.Println(filepath.Join(p.File.Dir(), p.File.LogicalName()))
 			}
@@ -119,7 +119,7 @@ expired.`,
 			return newSystemError("Error Processing Source Content", err)
 		}
 
-		for _, p := range site.Pages {
+		for _, p := range site.AllPages {
 			if p.IsExpired() {
 				fmt.Println(filepath.Join(p.File.Dir(), p.File.LogicalName()))
 			}
diff --git a/commands/multilingual.go b/commands/multilingual.go
new file mode 100644
index 000000000..68da7c96d
--- /dev/null
+++ b/commands/multilingual.go
@@ -0,0 +1,41 @@
+package commands
+
+import (
+	"sort"
+
+	"github.com/spf13/cast"
+	"github.com/spf13/viper"
+)
+
+var langConfigs map[string]interface{}
+var langConfigsList langConfigsSortable
+
+func readMultilingualConfiguration() {
+	multilingual := viper.GetStringMap("Multilingual")
+	if len(multilingual) == 0 {
+		langConfigsList = append(langConfigsList, "")
+		return
+	}
+
+	langConfigs = make(map[string]interface{})
+	for lang, config := range multilingual {
+		langConfigs[lang] = config
+		langConfigsList = append(langConfigsList, lang)
+	}
+	sort.Sort(langConfigsList)
+}
+
+type langConfigsSortable []string
+
+func (p langConfigsSortable) Len() int           { return len(p) }
+func (p langConfigsSortable) Less(i, j int) bool { return weightForLang(p[i]) < weightForLang(p[j]) }
+func (p langConfigsSortable) Swap(i, j int)      { p[i], p[j] = p[j], p[i] }
+
+func weightForLang(lang string) int {
+	conf := langConfigs[lang]
+	if conf == nil {
+		return 0
+	}
+	m := cast.ToStringMap(conf)
+	return cast.ToInt(m["weight"])
+}