Pythonize

[peteruhrig]

The current version looks more like a shell script than a Python script:

• importing ffmpeg but then it is actually using subprocess calls to the command line ffmpeg
• for the fsgan_cmd, an external script is called with "python3 ..." via a subprocess call
• same is true of the audio_cmd

I'm not sure all of this is problematic, and the command line interfaces of some of the dependencies used may be more stable than calling functions directly, but it sure looks very odd.