Add 'throttled' option for task

In our project we have a large number of unit test pages to run on sauce labs and when multiplied by our browsers to test on it creates a lot of jobs - this periodically leads to sauce labs reporting failure due to simply by overwhelmed by jobs.

The `throttled` option when used will release a maximum of `throttled` number of unit test pages to sauce labs concurrently which will cap the number of jobs at `throttled * browsers.length` and will mitigate this issue.