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_test.go

@@ -0,0 +1,128 @@
+// 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 (
+	"github.com/kyokomi/emoji"
+	"github.com/spf13/hugo/bufferpool"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+func TestEmojiCustom(t *testing.T) {
+	for i, this := range []struct {
+		input  string
+		expect []byte
+	}{
+		{"A :smile: a day", []byte(emoji.Sprint("A :smile: a day"))},
+		{"A few :smile:s a day", []byte(emoji.Sprint("A few :smile:s a day"))},
+		{"A :smile: and a :beer: makes the day for sure.", []byte(emoji.Sprint("A :smile: and a :beer: makes the day for sure."))},
+		{"A :smile: and: a :beer:", []byte(emoji.Sprint("A :smile: and: a :beer:"))},
+		{"A :diamond_shape_with_a_dot_inside: and then some.", []byte(emoji.Sprint("A :diamond_shape_with_a_dot_inside: and then some."))},
+		{":smile:", []byte(emoji.Sprint(":smile:"))},
+		{":smi", []byte(":smi")},
+		{"A :smile:", []byte(emoji.Sprint("A :smile:"))},
+		{":beer:!", []byte(emoji.Sprint(":beer:!"))},
+		{"::smile:", []byte(emoji.Sprint("::smile:"))},
+		{":beer::", []byte(emoji.Sprint(":beer::"))},
+		{" :beer: :", []byte(emoji.Sprint(" :beer: :"))},
+		{":beer: and :smile: and another :beer:!", []byte(emoji.Sprint(":beer: and :smile: and another :beer:!"))},
+		{" :beer: : ", []byte(emoji.Sprint(" :beer: : "))},
+		{"No smilies for you!", []byte("No smilies for you!")},
+		{" The motto: no smiles! ", []byte(" The motto: no smiles! ")},
+		{":hugo_is_the_best_static_gen:", []byte(":hugo_is_the_best_static_gen:")},
+		{"은행 :smile: 은행", []byte(emoji.Sprint("은행 :smile: 은행"))},
+	} {
+		result := Emojify([]byte(this.input))
+
+		if !reflect.DeepEqual(result, this.expect) {
+			t.Errorf("[%d] got '%q' but expected %q", i, result, this.expect)
+		}
+
+	}
+}
+
+// The Emoji benchmarks below are heavily skewed in Hugo's direction:
+//
+// Hugo have a byte slice, wants a byte slice and doesn't mind if the original is modified.
+
+func BenchmarkEmojiKyokomiFprint(b *testing.B) {
+
+	f := func(in []byte) []byte {
+		buff := bufferpool.GetBuffer()
+		defer bufferpool.PutBuffer(buff)
+		emoji.Fprint(buff, string(in))
+
+		bc := make([]byte, buff.Len(), buff.Len())
+		copy(bc, buff.Bytes())
+		return bc
+	}
+
+	doBenchmarkEmoji(b, f)
+}
+
+func BenchmarkEmojiKyokomiSprint(b *testing.B) {
+
+	f := func(in []byte) []byte {
+		return []byte(emoji.Sprint(string(in)))
+	}
+
+	doBenchmarkEmoji(b, f)
+}
+
+func BenchmarkHugoEmoji(b *testing.B) {
+	doBenchmarkEmoji(b, Emojify)
+}
+
+func doBenchmarkEmoji(b *testing.B, f func(in []byte) []byte) {
+
+	type input struct {
+		in     []byte
+		expect []byte
+	}
+
+	data := []struct {
+		input  string
+		expect string
+	}{
+		{"A :smile: a day", emoji.Sprint("A :smile: a day")},
+		{"A :smile: and a :beer: day keeps the doctor away", emoji.Sprint("A :smile: and a :beer: day keeps the doctor away")},
+		{"A :smile: a day and 10 " + strings.Repeat(":beer: ", 10), emoji.Sprint("A :smile: a day and 10 " + strings.Repeat(":beer: ", 10))},
+		{"No smiles today.", "No smiles today."},
+		{"No smiles for you or " + strings.Repeat("you ", 1000), "No smiles for you or " + strings.Repeat("you ", 1000)},
+	}
+
+	var in []input = make([]input, b.N*len(data))
+	var cnt = 0
+	for i := 0; i < b.N; i++ {
+		for _, this := range data {
+			in[cnt] = input{[]byte(this.input), []byte(this.expect)}
+			cnt++
+		}
+	}
+
+	b.ResetTimer()
+	cnt = 0
+	for i := 0; i < b.N; i++ {
+		for j := range data {
+			currIn := in[cnt]
+			cnt++
+			result := f(currIn.in)
+			if len(result) != len(currIn.expect) {
+				b.Fatalf("[%d] emoji std, got \n%q but expected \n%q", j, result, currIn.expect)
+			}
+		}
+
+	}
+}