fetchart: add 'sources' option

fix #1066

Fasten cover search by enabling engines selection and prioritisation.
It probably needs some more doc (to introduce the `*` wildcard) but I'm too sleepy right now for that. :full_moon_with_face:  
We should probably mention that this can be conveniently specified as a whitespace-separated line of source names. No need for the `['foo', 'bar']` JSON-like syntax.