Improvement of coverage for FFMPEG supported formats

[nschlia]

Discussed in #108

^{Originally posted by kimmot December 9, 2021}
First of all thank you very much for the great work. I tested ffmpegfs to implement a MP3 mirror of the music library in Daphile (www.daphile.com). It seems to do exactly what I hoped for.

I noticed however that it didn't support some audio formats that are still supported by ffmpeg e.g. DSF, DFF and APE. Also some file extension (e.g. flc) variants were not supported. Then I did a quick and dirty test by just renaming (without any format conversion) those files to supported format extensions (e.g. "mv test.dsf test_dsf.flac") and the result of ffmpegfs ("test_dsf.mp3") was usable i.e. its transconding worked from DSF to MP3.

I looked at the code (https://github.com/nschlia/ffmpegfs/blob/master/src/fuseops.cc#L382). I got an impression that "av_guess_format" is not very good for detection (I think it's mainly for ffmpeg supported output formats). I don't have really good idea how to improve the situation but could you consider implementing "-o"-option to provide a "whitelisted" file extensions that would be supported additionally.