Add path negation to music-transfer.py #32

It'd be nice to be able to have a transfer file with the following format:

/path/to/music-folder
!/path/to/music-folder/this-one-song-i-don't-want-to-sync.flac

This should also take precedence over including the given directory/file. eg. the following transfer file would not include any files:

/path/to/music-folder
!/path/to/music-folder

/path/to/another-file.flac
!/path/to/another-file.flac