Filename format of spectrograms to use a suffix

This is to make generated files more inline with other parts of the
script (log files, extracted artwork, etc).

The help/man page has been updated to reflect this change.  Other minor
tweaks have been made to the man page as well (spelling, grammar, etc).

Documented VORBIS_COMMENT tag field specification, including support for
tag fields to have whitespace as a valid character.