When an album has got a different artist for each track, the artist tag
of each track must not be overwritten by the one from the first track.

Keeping the artist tag for each track was previously possible with -a
option. But if you forgot this option, you could lose all artist tags.
It is safer to detect it automatically, and suggest that we keep all
artist tags in that case.

Remove all "Do you want to continue?" questions, since no command
requires so much precaution.

Update the year of the copyright.

Signed-off-by: Cédric Marie <cedric@hjuvi.lautre.net>