Istanbul Support

This PR adds istanbul report coverage support via something like this:

    run-browser test-file.js --port 3030 --coverage text --coverage html --coverage=cobertura

This changes a lot of code, and I found my usage of the `coverage` variable in index.js to be quite hacky, but I couldn't think of a better way to accomplish specifying the report type without re-writing the API and breaking backward compatibility. 

@Raynos if you can code review, I'd appreciate it. Would love to get this working with lint-trap, but that would require too many breaking style changes from FL's style. I tried to stick with FL's style as best as I could, but I found my self changing quite a bit that I may have deviated in places. Adding some unit tests would be nice. I'd be happy to add them, but then I would definitely want to do a bigger re-write and API change.