compare artists based on the slug
compare artists based on the slug

this is necessary because otherwise artists with different string
representations but the same slug would overwrite one another

this outlines more clearly the code duplication between the rst code
and the slugify function, something which can be fixed later.

beetbox/beets,sampsyo/beets,ibmibmibm/beets,jackwilsdon/beets,ibmibmibm/beets,SusannaMaria/beets,ibmibmibm/beets,jackwilsdon/beets,jackwilsdon/beets,shamangeorge/beets,beetbox/beets,shamangeorge/beets,shamangeorge/beets,SusannaMaria/beets,ibmibmibm/beets,sampsyo/beets,shamangeorge/beets,beetbox/beets,SusannaMaria/beets,sampsyo/beets,beetbox/beets,jackwilsdon/beets,SusannaMaria/beets,sampsyo/beets