use different criteria to priorize musicbrainz matches when resolving metadata

This MR adds a resolvers module which can be used to find the best match for a given item from a number of candidate items and changes the musicbrainz module to use the resolvers for annotating playlist items with musicbrainz metadata and optionally updating their existing fields.

Currently, two selector functions are implemented, one that will always use the best match, the other one uses a simple, interactive tty interface. For the latter, input and output must be files. For searching, as much info about an item as possible is fed into the musicbrainz api and once a match is found, as much info as possible will be written back. Therefore, the implementations of the annotate and resolve-ids commands would be identical, so the two are merged.