add --id3v23unsync option

[sktt]

This fixes https://github.com/robinbowes/flac2mp3/issues/58 by disabling id3v23_unsync for MP3::Tag by default. In case there are special cases I guess it could be re-enabled.

Also up for inverting the argument (eg. --no-id3v23unsync) to avoid regression.

However it may not even need an option and could always be disabled. What mp3 players don't support id3 tags? lame does not set the unsync flag as far as i can tell..