Include and extend support.

I have tested both client-side and run-time setups. It seems to work fine for simple testcases. I have not done any excludeJade testing, seems I am unsure whether it works at all even without my changes but my changes should not affect anything else other than the extended syntax support. I am aware that you would like to keep changes to Jade to minimal and my changes are perfectly scriptable if one wanted to make a script to convert jade to require-jade. Enjoy!