introduce registerFilter method to add filters to the jade runtime

Since metalsmith-jade uses the jade runtime located at ./node_modules/metalsmith-jade/node_modules/jade and not the project specific one at ./node_modules/jade it is not possible to register filters on jade directly.

Therefore the regiserFilter method was introduced. The method accepts the name of a filter and a filter factory method. The filter, factory method is passed the jade lang (require('jade')) the jade runtime  require('jade/lib/runtime')) and the jade filters (require('jade/lib/filters')) and should return a method which will be registered as a jade filter using the name passed as the first argument to the registerFilter method.

Would be happy if you could merge the PR and release 0.0.3.

Thanks