Raw audio stream muxer

This still needs a bit of cleanup, but it's working for me to stream or record the first audio stream in a service (i.e. a Radio station, but can also be used for recording/streaming audio from a TV channel) as a raw MPEG/AC3/AAC audio stream (i.e. no container at all).

NOTE: Please ignore the 6944425 commit, not sure how that got in here...
I would prefer "Audio-only MPEG elementary stream".