Doctype shortcut deprecation patch

### Changes

This patch does two things:

  1. restores the `!!!` doctype shortcut and doctype `5` modifier
  2. adds deprecation warnings for both.

### Rationale

Given how prevalent the use of `!!!` is, it would be helpful to have a buffer for informing people of this change, and updating applications before being permanently deprecated. Even then, a silent error seems more appropriate than throwing an error to stay consistent with the rest of jades templating behaviour. 

Please consider accepting this patch and tagging a `v1.0.3` release. It will assist in upgrading to Jade v1 in sintaxi/harp.

**all tests are passing**

thanks,
Brock