Add feature to add extractors to blacklist

[ghost]

• I've verified and I assure that I'm running youtube-dl 2018.03.03

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I don't want to able to play videos specific from specific sites, by names of extractors, for example:
--blacklist=xvideos,twitch

[dstftw]

This will introduce ambiguity for embeds of blacklisted extractors. I also don't see much sense in this feature. If you don't want to be able to download some videos don't download them at all.