[FEATURE] Be able to tell which sites should be filtered instead of applying to all by default.

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What title says. Its much easier and less demanding to tell the extension on which sites it should be active instead of the other way around. This makes the process simpler and also saves the user from having to add all the internet on the whitelist.
Also prevents occasional errors if the user is filtering a webpage he/she didn't want to and forgets that the extension is active.