small changes

[E-G-C]

-mux_all defaults to video file if not new video is created
-download return video path after download,

[elmoiv]

Thanks for your contribution!

I am planning now to convert videos without audio via ffmpeg too.
Problem is those videos have a codec that prevents you from sharing them through some applications and platforms.

[elmoiv]

Note that you should use the global formatting not Python 3.6+ formatting to make the project available for Python 3.x users.

Always use "{}".format(s) instead of f"{s}".
I already updated your pull request.

[E-G-C]

guilty 😔...haven't targeted anything lower than 3.6 in a while