Add support for multiple artist tags

[genericCandy]

I use beets with MPD, and every time a song / album is a collaboration between multiple people (leading to the artist field being A & B), it is classified as being from a new artist whose name is "A & B", which is obviously undesirable.

I thought that was an issue with MPD, but i found this issue on MPD, and I found no option in beets to change the way artists are tagged, so I think that there should be an option to have beets make multiple artist tags when there is a collab between multiple artists