Add support for YouTube thumbnails

[IBBoard]

Twitter web interface does "cards" for YouTube videos. Since Corebird already pulls Flickr and others independently of the data that Twitter supplies, it'd be nice to get media previews/thumbnails of YouTube videos.

Corebird is obviously unlikely to be able to play videos from YouTube inline, so it might need different behaviour to either not make it clickable or to make clicking the image go to the YouTube page instead.

(Probably overlaps #349, but I don't know why the reporter closed that one)