Add option to retrieve the metadata from MusicBrainz

[siebenstreich]

This idea was mentioned by diffycat at issue #10:
An option to retrieve the metadata from MusicBrainz would be fantastic!

To get the metadata, the MusicBrainz Release ID is needed (for multi-disc albums, I think it is extended by '#disc1' for example). The Release ID could be provided as a command line argument, as a tag, or as the content of a file.
Another option is to retrieve the metadata based on the discid, which I think is a hash value over the track lengths. However, the discid is potentially ambiguous, so I guess the Release ID approach is better.

Since it really would improve handling my music collection: I offer donating 50$ to the project if MusicBrainz support will be added to split2flac in the near future.