Make EMOJI_DATA user-extensible, custom shortcoes, custom emoji

[cvzi]

          It would be cool if there was a way to make these things user-extensible, I guess. Not sure what is extant for that sort of thing as of right now. Even just providing an extra little dict of `{":shortcut:": "<emoji>"}` in a setup call might be nice

Originally posted by @snus-kin in #268 (comment)