:fire: old API
Merge pull request #48 from github/fire-old-api

:fire: old gemoji API
Add API for adding new / editing existing emoji

- Emoji.create(raw) => yields to block
- Emoji.edit_emoji(emoji) => yields to block

The block forms are so that the list of aliases & unicode_aliases is
re-indexed after the update.
Add documentation note about images for added emoji

It comes down to: if you add new emoji, you must add images for them as
well. That's the whole point.