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
Started to increase coverage in helpers package, now at 74.9% of statements.
In the process, also a few minor changes have been applied to content.go.
* Content.go has undergone a formatting refactor regarding comments
* Unused function TruncateWords has been removed
* RenderingContext's "mmark" has been changed to use MmarkRender
* Content_test.go added to cover content.go's functionality
especially when the given `--source` path is a relative directory.
Also, when `--source` is specified, make WorkingDir an absolute path
from the very beginning, to be consistent with the case when `--source`
is not given. Otherwise, the function name helpers.AbsPathify(), which
prepends WorkingDir to a relative path, does not really make sense.
Fixes#1721
To allow the end users to disable any form of smart dashes
(LaTeX-style or not) while keeping the rest of Blackfriday
SmartyPants features.
Depends on https://github.com/russross/blackfriday/pull/190
"Add HTML_SMARTYPANTS_DASHES for toggling smart dashes"
to be accepted by Blackfriday developers.
Inserts a code tag into Pygments output with the language-info that is present when using client-side highlighting (useful for CSS hooks)
```html
<code class="language-go" data-lang="go">
```
closes#1490
