move filters to an external library

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` and does not install the dependencies itself, but lets you install the ones you need.