Add internal task config query interface lineman --config-enabled=sass

[davemo]

This will allow us to instrument conditional logic in the buildpack and shrink the time to build in the case people aren't using sass (or other things)

[jasonkarns]

👍

Though should we consider a more generic and future-proof CLI for managing options? Perhaps lineman config sass-enabled=true? It would make the CLI vs JSON config name match one-to-one. And make it possible in the future to manage other options in a consistent manner.