[Feature Request] Custom emoji support #30

In many forum and chat apps, you are able to create your own emojis, like

And you can have your own short code:

:parrot:

Configuration will be like:

remark().use(emoji, {
  customEmojis: {
    parrot: '![Parrot](https://cultofthepartyparrot.com/parrots/parrot.gif)'
  }
});