mutagen.id3 needs:
 * Detect tags in non-beginning locations, particularly if multiple
   separate version tags exist at the beginning of the file
 * Perhaps implement tag merging - "intelligently" merge information
   from all known tag formats

mutagen.apev2 needs:
 * General cleanup to fit the basic structure of the rest of Mutagen.

mutagen.ogg needs:
 * OggPage.replace can fragment pages poorly; maybe it should try to
   defrag the next two or three pages after the replaced ones.
