Add an MP3-tag style checker.
Fiddle about with "mp3taglint".

  - Add a word to the file comment.
  - Add disc number frame to the list of tag frames to check and
    rearrange said list.
  - Improve the error message that is shown when a file cannot be read.
"mp3taglint": Add another check and stuff.
Refactor usage instructions in Perl scripts.