[FR] "Blacklist" of sites where blocking is enabled

[cpatulea]

I don't normally use an ad blocker and I would like to use uBlock only on specific sites that are particularly aggressive with their ads.

This is the opposite of Whitelist - where only sites on the Blacklist are subject to blocking.

[gorhill]

See #177.

I personally will never implement such a feature, this runs counter to my principles.