Handle weird artist names

Like:
* [x] `Musica Nuda/voi Petra Magoni/ctb Ferruccio Spinetti/vib Daniele Di Gregorio/bat Balafon`
* [x] `Medeski/martin/wood`

Should be:
* `Musica Nuda, Petra Magoni, Ferruccio Spinett, Daniele Di Gregorio, Balafon` (and last.fm artist `Musica Nuda`)
* `Medeski, martin, wood` (and last.fm artist `Medeski`)