Adds support for aliases

```rb
require 'emoji/aliases'
Emoji.aliases_for('smile') #=> [...]
```

The use-case is to enhance the completion suggester for entering emojis.

This initial set of aliases is inspired by @muan's [emojis.json](https://github.com/muan/emoji/blob/cfb6f00843442fdbd97f6fe9e921447ac6d9663b/emojis.json).

This is :construction: :construction: since there are a couple of open questions:

* Is it OK to keep the list in YAML? It could be a plain text file just as well. I only want to keep it line-based for easier editing & diffs.
* Is it useful that this is an optional require, or should it come by default when you require `(g)emoji`?
* There is already a scheme for aliasing via symlinks. The following emojis already had multiple names:

<table>
<tr>
  <th></th>
  <th>names</th>
  <th>Unicode description</th>
</tr>
<tr>
  <td>:laughing:</td>
  <td>laughing, satisfied</td>
  <td>smiling face with open mouth and tightly-closed eyes</td>
</tr>
<tr>
  <td>:hankey:</td>
  <td>hankey, poop, shit</td>
  <td>pile of poo</td>
</tr>
<tr>
  <td>:boom:</td>
  <td>boom, collision</td>
  <td>collision symbol</td>
</tr>
<tr>
  <td>:+1:</td>
  <td>+1, thumbsup</td>
  <td>thumbs up sign</td>
</tr>
<tr>
  <td>:-1:</td>
  <td>-1, thumbsdown</td>
  <td>thumbs down sign</td>
</tr>
<tr>
  <td>:facepunch:</td>
  <td>facepunch, punch</td>
  <td>fisted hand sign</td>
</tr>
<tr>
  <td>:hand:</td>
  <td>hand, raised_hand</td>
  <td>raised hand</td>
</tr>
<tr>
  <td>:runner:</td>
  <td>runner, running</td>
  <td>runner</td>
</tr>
<tr>
  <td>:mans_shoe:</td>
  <td>mans_shoe, shoe</td>
  <td>mans shoe</td>
</tr>
<tr>
  <td>:shirt:</td>
  <td>shirt, tshirt</td>
  <td>t-shirt</td>
</tr>
<tr>
  <td>:bee:</td>
  <td>bee, honeybee</td>
  <td>honeybee</td>
</tr>
<tr>
  <td>:dolphin:</td>
  <td>dolphin, flipper</td>
  <td>dolphin</td>
</tr>
<tr>
  <td>:feet:</td>
  <td>feet, paw_prints</td>
  <td>paw prints</td>
</tr>
<tr>
  <td>:moon:</td>
  <td>moon, waxing_gibbous_moon</td>
  <td>waxing gibbous moon symbol</td>
</tr>
<tr>
  <td>:phone:</td>
  <td>phone, telephone</td>
  <td>black telephone</td>
</tr>
<tr>
  <td>:hocho:</td>
  <td>hocho, knife</td>
  <td>hocho</td>
</tr>
<tr>
  <td>:email:</td>
  <td>email, envelope</td>
  <td>envelope</td>
</tr>
<tr>
  <td>:memo:</td>
  <td>memo, pencil</td>
  <td>memo</td>
</tr>
<tr>
  <td>:book:</td>
  <td>book, open_book</td>
  <td>open book</td>
</tr>
<tr>
  <td>:boat:</td>
  <td>boat, sailboat</td>
  <td>sailboat</td>
</tr>
<tr>
  <td>:car:</td>
  <td>car, red_car</td>
  <td>automobile</td>
</tr>
<tr>
  <td>:izakaya_lantern:</td>
  <td>izakaya_lantern, lantern</td>
  <td>izakaya lantern</td>
</tr>
<tr>
  <td>:gb:</td>
  <td>gb, uk</td>
  <td>regional indicator symbol letter g + b</td>
</tr>
<tr>
  <td>:exclamation:</td>
  <td>exclamation, heavy_exclamation_mark</td>
  <td>heavy exclamation mark symbol</td>
</tr>
</table>

This PR adds another scheme for aliasing, and now we end up with 2 levels of aliasing, which I don't like. I would prefer to get rid of multiple aliases on the filesystem and just go the route with aliases kept in a data file. However, if we delete the filesystem aliases then we break backwards compatibility, since people might be using `:knife:` in their textareas and `knife.png` in their apps.