When a directory contains several audio files that are not part of an
album, it is useful to set or clean tags in a single file.
Moreover, for these single files, some tags - album, track number, and
disc number - don't make sense and must not be set.

Two new commands are available:
- clean-single
- set-single

Signed-off-by: Cédric Marie <cedric.marie@openmailbox.org>