Use conceal functionality to show emojis (:thumbsup: -> 👍)

[jakubbortlik]

Feature Description

I think it would be quite nice to use the conceal functionality in markdown buffers like the discussion tree and the comment composition buffers to visually replace the text like :thumbsup: with the actual Unicode symbols like :+1:.

@harrisoncramer, do you think this is something you'd be willing to include in the plugin? I haven't checked whether there is another plugin that does this.

I'm only afraid that adding 570 or more syntax rules for each emoji such as syntax match emoji_tada ":tada:" conceal cchar=🎉 could negatively affect responsiveness of the UI.