Fix support for multi-disc sets in cue files

Note:  you still need to quote disc and set numbers with
DISC and DISCC.  Ex:

PERFORMER "Keith Jarrett"
TITLE "Still Live"
DISC "1"
DISCC "2"
REM GENRE "Jazz"
REM YEAR "1986"

http://bugs.slimdevices.com/show_bug.cgi?id=17770