Add YouTube addon

[christianprescott]

Not sure what the status is of chorus1 with chorus2 support on the way, and I've seen your indication that it's coming in #87, but this is the YouTube addon I've been using for a while for simple searching and importing YT links to the playlist. Also adds video addons to those collected by app.addOns.getSources.

Some rough edges:

• Pagination in channels is clunky, uglies up the directory tree pretty quick
• Tree displays some items you can't interact with e.g. sign in, playlists header and their markdown
• Videos don't display title and thumbnail in the playlist until the video completes

[jez500]

Thanks for this, Chorus2 will has video addon support but this will be great until that gets released. I'll do an updated release with this included shortly