Outline of the changes made to this file:
* `handleMessage` now uses a `switch` statement instead of an unfolded `if`. I added two message types:
  * `worker_invoke` - pass a job to the slave worker, to defer CPU-intensive data processing tasks
  * `features_to_add` - get the amount of features that an addon registers. At the moment this thing is VERY specific to the BugzillaJS loader.