Multiplex m4v downloads with audio for more usable HD downloads

These changes prompt for an optional, additional audio file download if a video-only (no-audio) m4v download is chosen.

Both are downloaded and the files are mux'd with ffmpeg.  The original two downloads are deleted once the mux'd file is created.

This enables more useful file downloads at higher resolution (1920x1080) which were previously available only without audio.

The prompt appears only if ffmpeg is available in the system path and a m4v download is chosen.

This is based on the implementation in https://github.com/np1/pafy/pull/21 although I modified the ffmpeg arguments from that PR slightly because I was getting just audio output as the result of the mux.