Change the way to set AC/VC/MUX format.
Change the way to set AC/VC/MUX format.

* bind AC/VC/MUX format to output file group
* autodetect AC/VC/MUX format for each output file based on its
  extension
* allow user to override AC/VC for output via -ac/-vc options
* put user overrides for -ac/-vc in OUTAC/OUTVC variables
* add comment for async=4000 filter option
* use $OUT0AC for audio codec option in voldetect and final encoding
  phases
* append output group 0 options in voldetect phase
* use $OUT0VC for video codec option in final encoding phases

xrgtn/recode-vid,xrgtn/recode-vid