(flac2mp3): Don't need to int(track) now that the loop in main does it.

(flac2mp3): Don't need to int(track) now that the loop in main does it.

(Tags): New class that should be moved somewhere shared so fa-flacd
and others can use it.  Simplifies keeping track of per-disc and
per-track tags.

(get_tags): Build and return a Tags object instead of mucking about
with different tags in inconsistent (and broken!) ways.

(main): Oy, main needs to be split up.  Change the main loop to use
the Tags object, fix discnum-is-a-str-not-int bug, make track always
an int here (previously was sometimes str), correctly handle multiple
tracks per flac, and append PART to TITLE if present.