Feature/issue 165

Adds functionality that allows the user to whitelist certain potential trackers which need to be whitelisted to work. E.G. Youtube comments. This feature is already present in the chrome version but this would add it to the firefox version. CC @jsha for review. 
I think we can't be super-duper confident that url has :// in it somewhere (i.e. maybe we could get called with about:blank). I notice the call site has an nsIURI handy and turns it into a string with .spec. Maybe pass along the nsIURI? Then you should in theory be able to pull out host + port + path here more cleanly. Also, add JSDoc.