Config option to pass cookies to the browser

Attach to #6.

Added a `cookies` key to the configuration file, which allows cookies to be injected into/added to the browser (PhantomJS). The cookies key contains only the name, value, and domain, as the [phantomjs-node module](https://github.com/sgentle/phantomjs-node/blob/eb082f5a799964310b78af642ae3d155f185e6d5/shim.js#L4670) doesn't currently expose the same API as PhantomJS itself.

Updated the README file as well to reflect the new option.