Exposing jade object. Desirable for defining filters.

Hi. It would be nice to be able to define jade filters from gulpfiles. From what I've found it seems the way to do this is with something like `require('jade').filters.myFilter = function () { ... }`. So, to be able to define filters in `gulpfile.js` I would need to access that filters object, or just jade's main object.

I just added a reference to the jade object in your modules exports so it can be accessed with `require('gulp-jade').jade`. Please consider merging if you think this is desirable. Any reasons why this is not a good idea? Anyone know other way of defining filters?