Allow selection of source for genre information (ie - artist vs album)
Allow selection of source for genre information (ie - artist vs album)

If config is set to:

   lastgenre:
     source: artist

The genre will be fetched for the artist, rather than the album.  This
allows for filesystem org like:

   genre/artist/album

Currently defaults to previous behaviour for anything other than
`artist`

google-code-export/beets,google-code-export/beets,google-code-export/beets