Upgrade to ReSpec markdown support

Hi,
Could you please let me know if you are using ReSpec's markdown support?

The markup parser we were using was a few years out of date, so I've
updated it. However, before I release a new version of ReSpec, I want
to make sure that specs don't break.

One notable change, we no longer strip whitespace on `pre` elements,
so code looks nice :) See:
https://github.com/w3c/respec/issues/724#issuecomment-216155656

I've also switched on GitHub-flavored markdown, because it's the best.

Another notable difference: sections will not get an id, only headers
elements will (based on the <h*>'s content).  If you want a section to
have an id, please manually add it.

If you wish to review the changes + give feedback:
https://github.com/w3c/respec/pull/732

Lastly, the markdown parser is now part of our auto-update queue - so
it will be automatically updated with every new release 🎉

Kind regards,
Marcos