Support multi-valued tags for track artists

[ProfMoo]

Now that multi-valued tags are supported for album artists in v0.16, I think it'd be great to expand this functionality to track artists 😄.

From a user perspective, I'd be great to be able to find all the tracks that a certain artist was involved with (ex: Skrillex was featured on one song on J Balvin's album. Ideally, we could associate that will Skrillex in the db).

As an example, here is how it looks in Roon currently on the track "Scenario" by A Tribe Called Quest:

Musicbrainz link here, track #14. Each artist in Roon is individually navigable. Although I believe Roon is enabling this via Musicbrainz rather than via tags/folder structure.

[sentriz]

I would called this closed with #397 being merged. it's a little experimental at the moment but can be configured with the new GONIC_MULTI_VALUE_ARTIST setting

I'll be improving it again with a follow up PR