Adding test run data to html report

Closes #118

Also supports running the html report without coverage data.
[Element.classList](https://developer.mozilla.org/en-US/docs/Web/API/Element.classList) is pretty nice: `element.classList.remove('hide'); element.classList.add('show');`.

[document.querySelectorAll](https://developer.mozilla.org/en-US/docs/Web/API/Document.querySelectorAll) is similarly rad, allowing jquery-esque selectors -- not strictly necessary here, though!