Enable include/extend from root of project without prepending oodles of ../'s

Addresses the issue posed in this question
http://stackoverflow.com/questions/12905538/how-can-i-include-extend-a-jade-file-from-the-root-of-my-template-directory-in-e

File under sugar, but it makes my life easier. Note that I only use jade for static page
generation so I am not very concerned with speed of execution.

We can now write extensions of base types anywhere without prepending
a whole bunch of ../ action. If no file is found counting from the
cwd (where jade is run from), then it checks for the file relative
to the current .jade file being parsed.