Use getopt.gnu_getopt

If you were ever annoyed by starting Picard from the console with a
specific folder, noticing you forgot the `-d`, stopping Picard, pressing
arrow-up, adding the `-d` to the end of the line, only to notice that
this didn't help at all, this commit is for you because now the option
parsing doesn't stop after the first thing that's not an option.
Merge branch 'master' of github.com:musicbrainz/picard