Rhythmbox plugin to show how 'mainstream' each artist is

[ssssam]

We could generate a 'mainstream' percentage based on how many listeners an artist has on Spotify, Last.fm. There may be other data sources too. This could be shown in Rhythmbox to allow filtering music by how 'mainstream' it is.

[ssssam]

https://github.com/donaghhorgan/rhythmbox-plugins-rating-filters/blob/master/dev/RatingFilters.py gives an idea on how to do this