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. 
Style-wise, my preference for functions that take a function argument is like this (to mitigate the rightward march):

```
return doDependingOnIsPrivate("domainExceptions", function(store) {
  if (!store[location.host]) { return false; }
```