Improvements to the spec runner

This PR cherry pick the updates @hcatlin made to the spec runner in #206. There are two changes.

#### --unexpected-pass

The `--unexpected-pass` flag now produce significantly less noise making it easier than ever to find magically passing tests.

#### --nuke

The `--nuke` flag as added. When supplied it overrides the `expected_output.css` of failing specs with the output of the supplied engine.