feature(audioPassthrough): adding in basic audio passthrough functionality

Attempting to allow audio passthrough for defined codecs, per the existing ticket and my own needs. Usage is through `--audio-passthrough=CODEC,CODEC2`, and `--use-audio-passthrough` to simply passthrough all audio tracks. One additional feature crept in as well, the ability to preserve the number of audio channels in the source via the option `--audio-channels=SOURCE`.

NOTE: To keep the PRs clean, this does NOT include my fix for audio bitrate, channels and codec. I'll attempt to clean that up so my master includes both (as-in, is fully functional).