Added Vorbis (.ogg) audio format

I added the Vorbis audio format. I have success transcoding audio to Vorbis with this addition with FFMpeg compiled with libvorbis. Caveat: It can only transcode 2 channel (stereo) audio input.

I've tried to adhere PHP coding standards and run the files through php-cs-fixer.

Please let me know if I could do anything to improve this pull request. Thank you.