Add emoji rendering

Hi!

I added emoji rendering using `emoji-images` module. ( https://github.com/HenrikJoreteg/emoji-images )

This change enables github-like emoji notation like:
`:tropical_drink:` -> :tropical_drink:

This change is not completely same as github's one.
Github's emoji ignores notations inside code blocks, but this version renders them.
I prefer github's behaviour but implementing it seems to need some tricks.
So I avoided to do it at this point.

Thanks!