Add support for performing SASS transpilation for some files and keep those outside of the production css bundle

[dsebastien]

Goal: allow to have application SASS files that are NOT imported in main.scss and that should not be bundled in bundle.min.css. One use case for this are files that should be used with styleUrls in Angular 2. If those files don't exist in production then styleUrls cannot be used and thus per-component style isolation is not possible to implement.

Possible implementation:

• load all partial SASS files (i.e., starting with _) + main.scss
  • transpile those and bundle them in bundle.min.css
• perform a second pass where all non-partial SASS files with the exception of main.scss
  • transpile those in place (just like in dev)

Reference: dsebastien/modernWebDevGenerator#94