Added support for Emojis

Hi there I just added Emoji support in my app (osomtalk.com) over your library and I tought it would be nice to share the changes with you given that this could get marked closer to the current GFM.

I had to add a sub-module pointing to [gemoji](https://github.com/github/gemoji) soo it has the correct images but it could as well be without them(those are the ones used in github).

I had to add a very long string around 9kb, with the valid emojis (at least matched against the current gemoji set).

```
    emojis: ':100:1234:-1:+1:8ball:abcd:abc:ab:accept:aerial_tra......
```

I didn't implemented yet the use of unicode emojis `:u55b6:` for example because I would need yet another long string for validation.

Please let me know if you like it, or not.

Cheers!!