Feature: Allow audio_norm to properly retain ID3 information in MP3s

[iamdamosuzuki]

It should be possible to run audio_norm on MP3s without stripping the ID3 metadata

[eddycolloton]

It looks like there's a few python tools for doing just this.

1. One called replica: https://superuser.com/questions/436008/matching-id3-tags-to-existing-files
2. And another that uses a library called mutagen: https://stackoverflow.com/questions/7219871/copy-values-of-a-mp3-tag-to-another-tag-for-a-batch-of-mp3-files

I haven't tested either.

There's a bash tool called id3v2, but it looks like you may have to name the individual tags. It's available through homebrew so I'm going to give it a try.