[PrivacyBadger] Handling disqus and other sites

[Cooper Quintin]

I have written a proposal for how I think we should handle disqus,
youtube commenting, and other sites which the user may wish to whitelist
for its functionality.
The proposal is here:
https://github.com/EFForg/privacybadgerchrome/issues/137

I am going to start work on implementing the proposal for chrome,
however I would appreciate feedback from you all on it.  I won't do a
release until we all come to an agreement on how it should be handled.