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

helpers/emoji.go

@@ -0,0 +1,94 @@
+// Copyright 2016 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+package helpers
+
+import (
+	"bytes"
+	"github.com/kyokomi/emoji"
+	"sync"
+)
+
+var (
+	emojiInit sync.Once
+
+	emojis = make(map[string][]byte)
+
+	emojiDelim     = []byte(":")
+	emojiWordDelim = []byte(" ")
+	emojiMaxSize   int
+)
+
+// Emojify "emojifies" the input source.
+// Note that the input byte slice will be modified if needed.
+// See http://www.emoji-cheat-sheet.com/
+func Emojify(source []byte) []byte {
+
+	emojiInit.Do(initEmoji)
+
+	start := 0
+	k := bytes.Index(source[start:], emojiDelim)
+
+	for k != -1 {
+
+		j := start + k
+
+		upper := j + emojiMaxSize
+
+		if upper > len(source) {
+			upper = len(source)
+		}
+
+		endEmoji := bytes.Index(source[j+1:upper], emojiDelim)
+
+		if endEmoji < 0 {
+			break
+		}
+
+		nextWordDelim := bytes.Index(source[j:upper], emojiWordDelim)
+
+		if endEmoji == 0 || (nextWordDelim != -1 && nextWordDelim < endEmoji) {
+			start += endEmoji + 1
+		} else {
+			endKey := endEmoji + j + 2
+			emojiKey := source[j:endKey]
+
+			if emoji, ok := emojis[string(emojiKey)]; ok {
+				source = append(source[:j], append(emoji, source[endKey:]...)...)
+			}
+
+			start += endEmoji
+		}
+
+		if start >= len(source) {
+			break
+		}
+
+		k = bytes.Index(source[start:], emojiDelim)
+	}
+
+	return source
+
+}
+
+func initEmoji() {
+	emojiMap := emoji.CodeMap()
+
+	for k, v := range emojiMap {
+		emojis[k] = []byte(v + emoji.ReplacePadding)
+
+		if len(k) > emojiMaxSize {
+			emojiMaxSize = len(k)
+		}
+	}
+
+}