Lazy calculate WordCount, ReadingTime and FuzzyWordCount

This avoids having to execute these expensive operations for sites not using these values.

This commit sums up a set of wordcounting and autosummary related performance improvements.

The effect of these kind of depends on what features your site use, but a benchmark from 4 Hugo sites in the wild shows promise:

```
benchmark           old ns/op       new ns/op       delta
BenchmarkHugo-4     21293005843     20032857342     -5.92%

benchmark           old allocs     new allocs     delta
BenchmarkHugo-4     65290922       65186032       -0.16%

benchmark           old bytes      new bytes      delta
BenchmarkHugo-4     9771213416     9681866464     -0.91%
```

Closes #2378

2 files changed, 27 insertions, 7 deletions

diff --git a/helpers/content.go b/helpers/content.go
index 9d35675f7..f3d8bd94f 100644
--- a/helpers/content.go
+++ b/helpers/content.go
@@ -138,19 +138,28 @@ func StripHTML(s string) string {
 	// Walk through the string removing all tags
 	b := bp.GetBuffer()
 	defer bp.PutBuffer(b)
-
-	inTag := false
+	var inTag, isSpace, wasSpace bool
 	for _, r := range s {
-		switch r {
-		case '<':
+		if !inTag {
+			isSpace = false
+		}
+
+		switch {
+		case r == '<':
 			inTag = true
-		case '>':
+		case r == '>':
 			inTag = false
+		case unicode.IsSpace(r):
+			isSpace = true
+			fallthrough
 		default:
-			if !inTag {
+			if !inTag && (!isSpace || (isSpace && !wasSpace)) {
 				b.WriteRune(r)
 			}
 		}
+
+		wasSpace = isSpace
+
 	}
 	return b.String()
 }
diff --git a/helpers/content_test.go b/helpers/content_test.go
index 82af70f8f..22c81005f 100644
--- a/helpers/content_test.go
+++ b/helpers/content_test.go
@@ -34,11 +34,22 @@ func TestStripHTML(t *testing.T) {
 	}
 	data := []test{
 		{"<h1>strip h1 tag <h1>", "strip h1 tag "},
-		{"<p> strip p tag </p>", " strip p tag \n"},
+		{"<p> strip p tag </p>", " strip p tag "},
 		{"</br> strip br<br>", " strip br\n"},
 		{"</br> strip br2<br />", " strip br2\n"},
 		{"This <strong>is</strong> a\nnewline", "This is a newline"},
 		{"No Tags", "No Tags"},
+		{`<p>Summary Next Line. 
+<figure >
+    
+        <img src="/not/real" />
+    
+    
+</figure>
+.
+More text here.</p>
+
+<p>Some more text</p>`, "Summary Next Line.  . More text here.\nSome more text\n"},
 	}
 	for i, d := range data {
 		output := StripHTML(d.input)