Add offlinetest make target
Remove test/write_info_json

This is now covered by every single test_download testcase anyways :)
Improve and test ffmpeg version detection
[Makefile] Ensure that offline test really is offline
[YoutubeDL] Make postprocessors declarative

Instead of having to configure PPs in code, this allows us and embedding programs not to worry about imports or finer details, similarly to how we handle IEs.
[YoutubeDL] Add declarative version of progress hooks