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

If config is set to:

```yaml
lastgenre:
  source: artist
```

The genre will be fetched for the artist, rather than the album.  This allows for filesystem org like:

    genre/artist/album

Where an artist's albums might be in various genres, but you'd prefer to keep them together, under the artist's primary genre.

Currently defaults to previous behaviour for anything other than `artist`.