Add mapping as follows:
nmm:mbTrackID   -  GRL_METADATA_KEY_MB_TRACK_ID
nmm:mbArtistID   -  GRL_METADATA_KEY_MB_ARTIST_ID
nmm:mbRecordingID   -  GRL_METADATA_KEY_MB_RECORDING_ID
nmm:mbReleaseID   -  GRL_METADATA_KEY_MB_RELEASE_ID
nmm:mbReleaseGroupID   -  GRL_METADATA_KEY_MB_RELEASE_GROUP_ID

MusicBrainz Release ID and Release Group ID are supported in
grilo since 0.3.8

#48