Add MP4 replay gain tag support, providing you have libMP4v2.

If you don't have libMP4v2 and want MP4 tag reading support, you either need to install it or extend metadata/m4a to read freeform data (ie: starting with ----).

I personally have no interest in working on metadata/m4a when metadata/mp4 works perfectly well. However, if someone implements freeform data reading in metadata/m4a, I'm happy to do the rest of the work for reading replay gain tags.