Allow application of arbitrary ffmpeg filters

[deefdragon]

I have what I fully recognize is a very niche issue. I have a playlist of songs that I listen to at 2x speed. Would it be possible to add an option that would allow the user to apply an arbitrary ffmpeg filters (like 2x speed to the media after download) to a source playlist/channel etc so I dont have to apply those manually?

Mind, being able to apply speed up alone would make it nice to watch on sources that don't have native speed controls.

[meeb]

This might be easier handled with a post-processing shell script rather than adding in a bunch of options specifically for this use case. I'd review a PR though if you wanted to submit one.