Backfill

This allows `backfill: yes` to be specified on the emit_series plugin, which will cause the plugin to iterate backwards over all seasons up to the current season and emit every episode. It will continue to retry episodes that had no acceptable matches as well. The idea here is to allow the user to add a series that's currently in the Nth season, and have the plugin work backwards and attempt to download every episode up to the current one.

Along with this is an `allow_backfill: yes` option to be specified on a per series basis, this setting allows backwards episode advancement without restrictions, and should be used on any season the user is attempting a complete backfill on.

I also refactored `get_latest_download` to be `get_latest_release` with optional `season` and `downloaded` parameters to allow more flexibility.

And lastly, I added a crapload of tests because there were none for the emit_series plugin and that's bad.