Selenium: add code coverage for selenium tests

[philschatz]

This injects the JS built by webpack with coverage recording and then the selenium mocha tests extract the coverage data out at the end of each test using the __coverage__ global that istanbul fills. refs #931

Example coverage Report

To run

Run npm run test-integration. This command:

1. builds the JS files with coverage recording sprinkled in
2. serves the files at http://localhost:8000
3. runs selenium tests while recording coverage information to /coverage-selenium.json
4. generates an HTML and .lcov report in /coverage/

TODO

• save the __coverage__ data to a file and then generate a report

Thoughts @openstax/tutor-fe ?