MP3 support

STR:

1. Take two MP3 files which differ slightly (such as in their metadata).
2. Compare them using diffoscope.
3. View the resulting differences.

Expected: Diffoscope supports MP3 files and I get a useful comparison view.

Actual: I get only a hexdump (which has many differences).

While not important for my daily work, this case up today and having diffoscope support here would have been useful.