Warning API or update Jade and corresponding tests for 1.0.2

Jade deprecated `!!!` for `doctype` in 1.0.0, [which is unfortunate](https://github.com/visionmedia/jade/pull/1270). It was also decided not to make it a warning, like with `script.`, but actually throw an error.

I don’t necessarily think this should get merged, but we’ll have to deal with it at some point. It’s going to break every app written in Jade since we were advocating the `!!!` shorthand for `doctype` (I didn’t even know that was shorthand, honestly).

If anything, it might indicate we need some way to throw our own warning pages, just like for errors. Then, when breaking API changes happen again, either in Harp like with `_data`, or in the preprocessors like the LESS and Bootstrap 2 one, or this `!!!` one, or whatever else, we could fail more gracefully. It would actually also be great for missing libsass features, too.