passthrough_extensions counterpart to include_extensions

[ToxicFrog]

The default behaviour is to transcode everything that isn't already the target format. By using include_extensions, you can restrict this to a list of specified extensions. In my case, however, I have a bunch of formats that the software I'm using supports, and I want to transcode everything else and leave those alone. At the moment I have to do this by using include_extensions for every format in use except the ones that don't need transcoding, which is (so far) 16 include_extensions options; it would be a lot more convenient to be able to say "pass through mp3, opus, ogg, and flac unaltered and transcode anything else you recognize".