Fix audio streams

[tomhense]

• Fixes issue Output file contains two audio streams #10
• Makes encoding much faster by using ffmpeg and encoding in parallel

[plusuncold]

Hi, this is great - a solid improvement.

Is there's a requirement to remove pydub to make this work? Only that the poetry.lock needs to be updated in lock-step. So either you'll need to update that file using poetry lock or take out the change to pyproject.toml.

One other change is could you move the ffmpeg call back to multiline? It makes the linter happy and helps on small screens.

[tomhense]

There is no requirement to remove pydub it's just technically not needed anymore. But I put it back into pyproject.toml in the lastest commit. The ffmpeg call is now also a bit prettier formatted.