Custom Parser Override

This commit expands on pull request:
https://github.com/visionmedia/jade/pull/1017
To allow for a custom parser override.  However, since the parser itself instantiates new Parsers, the change from pull request by@sax1johno must also be brought into parser.js to fully allow for a custom parser override.

ref: https://github.com/visionmedia/jade/commit/e4b0fd6a357948055c80c7f1535db70396277565