DSL code to handle SASSC compiler options


(build <arachne-id> compiler-opts)
Define an Asset transducer component which builds SASSC.

Arguments are:

- arachne-id (optional): the Arachne ID of the component
- compiler-options: A SASSC compiler options map. The only difference is that options which specify
                    paths (:output-to, :output-dir,etc.) will relative to the asset fileset rather
                    than the process as a whole.

Returns the entity ID of the newly-created component.