Labels plugin

This plugin lets you attach arbitrary text labels to your music and use them to filter the library.

Adds a new command ``labels`` which either filters by labels or attaches them to items (or albums) matching a query.

Labels are stored in a pretty straight forward many-to-many tagging system in the database using three new tables: `labels` which contains the label strings + join tables `labels_items` and `labels_albums`.

Just has the most basic features for now, but I thought this might be a nice way to generate playlists and other things.