Add support for URLs relative to context root

Setting `RelativeURLs` to `true` will make all relative URLs in the site *really* relative.

And will do so with speed.

So:

In `/post/myblogpost.html`:

`/mycss.css` becomes `../mycss.css`

The same in `/index.html` will become:

`./mycss.css` etc.

Note that absolute URLs will not be touched (either external resources, or URLs constructed with `BaseURL`).

The speediness is about the same as before:

```
benchmark                    old ns/op     new ns/op     delta
BenchmarkAbsURL              17462         18164         +4.02%
BenchmarkAbsURLSrcset        18842         19632         +4.19%
BenchmarkXMLAbsURLSrcset     18643         19313         +3.59%
BenchmarkXMLAbsURL           9283          9656          +4.02%

benchmark                    old allocs     new allocs     delta
BenchmarkAbsURL              24             28             +16.67%
BenchmarkAbsURLSrcset        29             32             +10.34%
BenchmarkXMLAbsURLSrcset     27             30             +11.11%
BenchmarkXMLAbsURL           12             14             +16.67%

benchmark                    old bytes     new bytes     delta
BenchmarkAbsURL              3154          3404          +7.93%
BenchmarkAbsURLSrcset        2376          2573          +8.29%
BenchmarkXMLAbsURLSrcset     2569          2763          +7.55%
BenchmarkXMLAbsURL           1888          1998          +5.83%

```

Fixes #1104
Fixes #622
Fixes #937
Fixes #157

2 files changed, 16 insertions, 6 deletions

diff --git a/target/file.go b/target/file.go
index ea023d2bb..01037a72b 100644
--- a/target/file.go
+++ b/target/file.go
@@ -16,6 +16,11 @@ type Translator interface {
 	Translate(string) (string, error)
 }
 
+// TODO(bep) consider other ways to solve this.
+type OptionalTranslator interface {
+	TranslateRelative(string) (string, error)
+}
+
 type Output interface {
 	Publisher
 	Translator
diff --git a/target/page.go b/target/page.go
index 57e3a300b..98e6dc8d4 100644
--- a/target/page.go
+++ b/target/page.go
@@ -32,10 +32,18 @@ func (pp *PagePub) Publish(path string, r io.Reader) (err error) {
 }
 
 func (pp *PagePub) Translate(src string) (dest string, err error) {
+	dir, err := pp.TranslateRelative(src)
+	if err != nil {
+		return dir, err
+	}
+	if pp.PublishDir != "" {
+		dir = filepath.Join(pp.PublishDir, dir)
+	}
+	return dir, nil
+}
+
+func (pp *PagePub) TranslateRelative(src string) (dest string, err error) {
 	if src == helpers.FilePathSeparator {
-		if pp.PublishDir != "" {
-			return filepath.Join(pp.PublishDir, "index.html"), nil
-		}
 		return "index.html", nil
 	}
 
@@ -43,9 +51,6 @@ func (pp *PagePub) Translate(src string) (dest string, err error) {
 	isRoot := dir == ""
 	ext := pp.extension(filepath.Ext(file))
 	name := filename(file)
-	if pp.PublishDir != "" {
-		dir = filepath.Join(pp.PublishDir, dir)
-	}
 
 	if pp.UglyURLs || file == "index.html" || (isRoot && file == "404.html") {
 		return filepath.Join(dir, fmt.Sprintf("%s%s", name, ext)), nil