fetchart: Fetch cover art from discogs

[achmetinternet]

I have the experience that discogs usually provides the best artwork at least for the music I need to tag and Amazon either does not have an image or it is..frankly speaking...crap.

[sampsyo]

I imagine this feature will depend on the discogs library, in which case this should be made an optional dependency. This way, people can still use fetchart without installing anything but can install the client library to find more art.

Also, it would be great if this integrated with the discogs plugin by detecting when an album is tagged from discogs (by looking at its source URL) and avoiding a second fuzzy search—just directly look up the art for that exact release.