Support rating tag

Related issue: https://github.com/leetreveil/musicmetadata/issues/137

Parse rating tags and map it to a common tag. In line with MusicBrainz Picard, normalize the common tag to a scale from 0 to 5.0:

            elif frameid == 'POPM':
                # Rating in ID3 ranges from 0 to 255, normalize this to the range 0 to 5
                if frame.email == config.setting['rating_user_email']:
                    rating = string_(int(round(frame.rating / 255.0 * (config.setting['rating_steps'] - 1))))
                    metadata.add('~rating', rating)

_source: https://github.com/metabrainz/picard/blob/master/picard/formats/id3.py_

[Borewit]

Added in v2.0.0

[Borewit]

Thanks for your feedback gents. Let’s enable rating support.