Allow passthrough of original video track

[Eric-Vignola]

Basically this link from stack overflow describes what I want to achieve.

I wish to add an audio track to an existing movie file without re-encodeing the video. A straight up ffmpeg command to do this would be: ffmpeg -i original_video.mp4 -i new_audio.wav -map 0:v -map 1:a -c:v copy video_with_audio.mp4

The new video would have an exact unaltered copy of the source + an new encoded audio track. I tried to achieve this directly with moviepy by adding ffmpeg_params to the write_videofile method of VideoFileClip but that didn't work.

As a feature, I think it would be nice to (when possible) have a way to use moviepy without necessarily re-encoding a video source.