Added conditional to `program.write_m3u()` to
ignore skipped tracks.
Added skipped_tracks support to the `Program` and
`image.ImageVerifyTask` classes to avoid crashing
when a file for a skipped track doesn't exist.
Added conditional to `accurip.calculate_checksums`
to check if a path exists before trying to calculate
checksums, this prevents `accuraterip-checksum.c` from
emitting an error message (`sf_open failed!`) when a
path doesn't exist (as when a track is skipped).
Signed-off-by: blueblots <63152708+blueblots@users.noreply.github.com>