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. 
In this case I plan to add the commented code back in later. But right now pb is architechted in such a way that I can't support this feature. Is there a best practice for what I should do in this case? Should I remove the code and leave a TODO note, or a link to the appropriate github issue?