Bug fix!!! Oops... 3.3.26 flac > flac had issue.

Many thanks to issue #9 who pointed out that the small changes that made flac to flac conversion possible inadvertently broke flac to lossy conversions, which was obviously a bad thing.

Also added opus type to --ffmpeg, to make it complete for the free audio codecs. Rarely used, but now you can.

Used this mistake to refactor the validations a bit more to make it easier to avoid such mistakes in the future, and also updated the man page to add some new options and features which were not completely documented, or documented inconsistently.