move filters to an external library

Rebase of #857

This makes filters just use an external library. It uses transformers which supports all the same filters as jade so there's no loss of functionality. There are loads of additional filters that are then supported, which I think is really useful.

We could move `transformers` and `coffee-script` into `devDependencies` and let people install them only if needed?

P.S.` transformers` works like `consolidate.js` and does not install the dependencies itself, but lets you install the ones you need.