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.
Change emoji creation API to take name as 1st argument

It's more likely that someone will want to add a new emoji with a custom
image rather than a character that has a Unicode representation.

Also move the VARIATION_SELECTOR_16 logic outside of Emoji::Character
since it doesn't need to be concerned with it.
Add ability to use `Emoji.create` without a block
Merge pull request #49 from github/edit-emoji

Add API for adding new / editing existing emoji