Enable setting "config.hidepassed" from URL params

Loading a QUnit test suite with `?hidepassed` appended to the URL is useful for CI services that take a screenshot of the completed test suite upon completion in various browsers, for example.

Fixes #457

/cc @jzaefferer
What is not clear? If the checkbox is checked, the value of this field should be its `value` attribute or `true` by defualt. I think the code is pretty self-explanatory and doesn't need stating the obvious.