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. 
This export style is inconsistent with the rest of the file, and with e.g. privateBrowsing.js. Probably best is to adopt local style and assign directly exports.syncPreloads = function() {...}.

In the long run if you decide to consolidate export styles, my personal favorite is to put all exports in a block at the end of the file, e.g.

```
exports = {
  syncPreloads: syncPreloads,
  ...
}
```