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. 
It seems like these reloads are in the wrong place since they can get called multiple times when there are multiple whitelistDomains. Probably better to put outside the loop, if (msg.action !== 'not_now') { activeTab.reload() }