Track individual episode playback

[thomaswilburn]

• Add a new Table for tracks
• Add ability to perform LINQ-ish batch operations on tables (e.g. tableInstance.get.where(x => x.listened) or tableInstance.delete.where(r => r.feed == feed)). Behind the scenes this is probably just a regular iterator, I don't particularly want to add supplemental indexes to the tables.
• Remember the playback metadata object from the player, basically.