Add console.assert and console.debug (+ question about console methods)

(Initially via https://github.com/tmpvar/jsdom/issues/926, while porting over https://github.com/assaf/zombie/blob/master/src/zombie/console.coffee)

Unless I'm missing something, it doesn't look like any console methods other than `console.error` output anything in jsdom ([all point to this code path](https://github.com/tmpvar/jsdom/blob/master/lib/jsdom/level1/core.js#L868)). My questions are:

- If console output isn't currently exposed anywhere, would providing stubs for all console methods be sufficient?
- Should there be a way to capture console output for better testing? (and maybe also to expose externally?)

I'll add to this PR based on feedback.