Add emoji support

This uses the Emoji map from https://github.com/kyokomi/emoji -- but with a custom replacement implementation.

The built-in are fine for most use cases, but in Hugo we do care about pure speed.

The benchmarks below are skewed in Hugo's direction as the source and result is a byte slice,
Kyokomi's implementation works best with strings.

Curious: The easy-to-use `strings.Replacer` is also plenty fast.

```
BenchmarkEmojiKyokomiFprint-4  	   20000	     86038 ns/op	   33960 B/op	     117 allocs/op
BenchmarkEmojiKyokomiSprint-4  	   20000	     83252 ns/op	   38232 B/op	     122 allocs/op
BenchmarkEmojiStringsReplacer-4	  100000	     21092 ns/op	   17248 B/op	      25 allocs/op
BenchmarkHugoEmoji-4           	  500000	      5728 ns/op	     624 B/op	      13 allocs/op
```

Fixes #1891

docs/content/overview/configuration.md

@@ -99,6 +99,9 @@ Following is a list of Hugo-defined variables that you can configure and their c
     disableRobotsTXT:           false
     # edit new content with this editor, if provided
     editor:                     ""
+    # Enable Emoji emoticons support for page content.
+    # See www.emoji-cheat-sheet.com
+    enableEmoji:				false
     footnoteAnchorPrefix:       ""
     footnoteReturnLinkContents: ""
     # google analytics tracking id

docs/content/templates/functions.md

@@ -413,6 +413,14 @@ These are formatted with the layout string.
 e.g. `{{ dateFormat "Monday, Jan 2, 2006" "2015-01-21" }}` → "Wednesday, Jan 21, 2015"
 
 
+### emojify
+
+Runs the string through the Emoji emoticons processor. The result will be declared as "safe" so Go templates will not filter it.
+
+See the [Emoji cheat sheet](http://www.emoji-cheat-sheet.com/) for available emoticons.
+
+e.g. `{{ "I :heart: Hugo" | emojify }}`
+
 ### highlight
 Takes a string of code and a language, uses Pygments to return the syntax highlighted code in HTML.
 Used in the [highlight shortcode](/extras/highlighting/).