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. 
For my own edification: In Firefox, will require() run the same file multiple times if you call it multiple times? Also, is this still called the page-mod UI? I saw you had some code with the pageMod name commented out.