gjay integration

Here's an idea, gjay integration: http://gjay.sourceforge.net/

It would be great to be able to use beets for assigning colours to tracks, and to call gjay to create playlists using those colours.
Shouldn't be too hard to implement since gjay stores its data in xml files.

No files, tests, or entry points are named. Start by examining beets' existing playlist and metadata integrations, then read gjay's XML format and determine how colors and playlist generation should connect. Done means a defined, tested integration that assigns track colors and invokes gjay to create playlists.

Written by the indexing model from the issue text.