Add target bitrate mode to streaming_config

[bctcvai]

The streaming config could use an overhaul to add target bitrate as an option.

Whilst we are in there, it might be worth just exposing a map to ffmpeg options given its generally admins vs. casual users making these configurations. There could be a map for "per resolution" arguments; its potentially dangerous to allow for too much customization on the output file, at least for the streaming subsection, because the output format needs to be fairly specific.