Lyrics plugin groups songs by 'artist' rather than 'albumartist'

[abra3607]

Problem

When you use the lyrics plugin to create rst files with the lyrics themselves, the resulting files are grouped by the items' artist field rather than albumartist, which would arguably be more reasonable. When an artist has many collaborations where for a song the two fields are different, it may lead to situations like this:

after a three line change (commit link):

Let me know if you want me to create a push request with this.

[sampsyo]

Good point! Yes, this would be useful as a PR. (Especially if you include a changelog entry.) Thanks! ✨