processing: Add Compass and Sass processors. The Sass processor is similar to the Less processor, i.e. it tries to be part of the structured pipeline processing by using the mapfile produced by the Sass compiler in order to provide a list of dependencies. The Compass processor is completely acting outside of the pipeline, so the server won't know what's up to date and what's not. It's expected that the user will run `compass watch` to keep things up to date. However, it will require to pass the server's cache directory to put things in, so we'll need to add some easy way to get that path for the user.