Enable Playlists/Artists/Albums/Tracks functionality with Airsonic library

[dberg81]

Iris currently integrates with Mopidy-Spotify and Mopidy-Local for built-in features of Playlist/Artist/Albums/Tracks.

Would it be possible to add support for these features with Airsonic/Subsonic libraries? Pointing to a centralized music database application would alleviate some of the headaches out of managing redundant copies of Mopidy-Local libraries across Mopidy hosts, and would eliminate the need for setting up NFS shares, user rights, etc to the local library on the Mopidy hosts.