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`
Add documentation for lastgenre source
Merge pull request #88 from pdf/lastgenre_source_selection

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