Add emoji support

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