This adds support for reading metadata from Opus files. The metadata config is copied from OggVorbis with just the 'track' field name changed.
Also make sure get_audio_file_data() in music/utils.py doesn't crash if no bitrate information is set, since mutagen doesn't add a bitrate to Opus info objects.
Fixes #244 (closed).
Signed-off-by: Toke Høiland-Jørgensen email@example.com