Drop most of the extractor to make the project more manageable

[TheFrenchGhosty]

YouTube-DL has been in maintenance mode for years, and I think it's because there is just too much stuff in it.

In my opinion a proper fork of it should drop most of it's extractor, and only focus on public video website (youtube, dailymotion, vimeo).

It would be far better if it could support external extractor, that don't need to be taken care by the main developper.

YouTube-DL should only take care of the basis and people should add other features according to what they want.

PS: Thank you for the fork, I hope you will be able to maintain it for years.