albumartist tag support

[andrius4669]

mpdscribble currently doesn't support sending albumartist tag

[andrius4669]

according to https://www.last.fm/api/scrobbling there's albumartist tag

[andrius4669]

uhhh apparently mpdscribble uses https://www.last.fm/api/submissions which doesn't have that tag support

This would be great, as it would allow more accurate submissions to last.fm. Especially with mopidy-spotify. Rather than Artist1;Artist2;Artist3 - Title, it could submit AlbumArtist - Title for example.