Support writing the audio stream back into video

[Ahuge]

This PR remuxes the audio stream back into the output video.

When output_type == video, we can support adding back the audio stream from the original video input.

It introduces a new convert_video flag passthrough_audio: bool = True.

[Ahuge]

I don't know if you are accepting PRs on this repo. I just found this functionality useful and wanted to offer it upstream