Add support for MP3 lyrics tag - USLT

Hello, this is a pretty simple change to add support for MP3 files with embedded lyrics.  The USLT tag is for unsynchronized lyrics, of the same format internally as comments, and it works ok with the MP3s I have with lyrics.  Also considered the SYLT tag, for synchronized lyrics, and has slightly more involved format.  Since I didn't have any MP3s with that tag, I did not add it.