Add ability to provide additional folders in the stylesPath for SASS compilation

[habdelra]

We use the bourbon SASS library in our application (which is installed via Bower). However, there isn't a means by which we can add additional folders to the stylesPath on https://github.com/glimmerjs/glimmer-application-pipeline/blob/master/src/broccoli/glimmer-app.ts#L331 without extending the GlimmerApp class. It would be nice to be able to pass in an array of additional style paths (so that we can add bourbon scss assets from its bower_components folder) that could be concat'ed to the stylesPath.

[rwjblue]

We are in the process of updating the build a pipeline to call hooks much more similar to a standard ember apps pipeline. This would allow ember-cli-sass to continue to function without needing to modify the build pipeline manually.