Custom font family for HTML progress indicator

Uses `window.getComputedStyle` to get the font family from `div#mocha`.

Example:

```html
<style>
  #mocha { font-family: "Source Sans Pro"; }
  #mocha .test pre { font-family: "Source Code Pro";  }
</style>
```

This is fully backwards compatible.
@boneskull Cause of this one `window.getComputedStyle(root).fontFamily`