add libcdio backend

[JonnyJD]

libcdio support was requested by sbontrager on the MB forums.

cd-info (part of libcdio) does read ISRCs just fine (not only in CD-TEXT) and gives something like this:

...
Media Catalog Number (MCN): 0000000000000
TRACK  1 ISRC: USXH21200030
TRACK  2 ISRC: USXH21200031
TRACK  3 ISRC: USXH21200032
...

There is also a python library pycdio, but I couldn't find anything of much help there. We should probably just parse cd-info output.