Version 2: Add support for running lighthouse tests locally

[mattheu]

I've made a few improvements to this.

• Option to run tests using a locally installed version of lighthouse. A big limitation right now is that it can only check publicly facing pages as it relies on the Google Pagespeed API to get results. This change means that you can run the exact same test with a locally installed version of lighthouse. In my testing, I'm not really seeing more noticable variance in results using this over pagespeed which was a big reason I used the API previously. The downside is that it can't run tests in parallel which the pagespeed API can do so is slower for a lot URLs. Select the method using the engine property in the config. Either pagespeed or lighthouse.
• Allow for configuration per group. Previously supported groups of URLs e.g. production, staging. But can't specify config options per group. I've restructured the config to introduce groups as a top level property allowing you to specify a full config for different groups of URLs. The config is merged with the top level one. This is a breaking change... hence bumping the version number, but is both simpler and more powerful.