[Feature request] Add Argument to set the post processor process priority

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In some case you don't want to run the post processor with normal priority. For example you do other stuff on the machine or there are other services.
Since the most solution (I checked only ffmpeg, since which it is what I use) in the web says you can't set it over the post processor args, you have to run it with another priority. It leads me, that it is responsible by the invoker, which mean youtube-dl.