Run automatic Lighthouse reports for pull reqeusts

[openjck]

We can either use the Lighthouse bot directly, or integreate Lighthouse with another CI service (see #79).

@pdehaan says more in #95:

Interestingly, it looks like Lighthouse as a CI bot which would run automatically for each PR: https://github.com/ebidel/lighthouse-ci

Or if we end up wiring up Circle-CI, we could build+start a production server on port 3000, and then use the lighthouse CLI:

lighthouse http://localhost:3000 --output-path=./lighthouse.json --output=json -GA --quiet

That should create a "./lighthouse.json" file which you could drag-n-drop it onto https://googlechrome.github.io/lighthouse/viewer/ to view the report.