Add phantomas perf monitoring grunt task

* Adds a grunt task to run [phantomas](https://github.com/macbre/phantomas) performance monitoring against the homepage and /firefox/new/ on www.allizom.org
* Outputs & updates pretty graphs & json data when run locally to */phantomas/

This is just a first step, we still need to figure out how to make this data more easily available to everyone when run (e.g. when we push to stage).