Process all SASS files that don't start with an underscore

The build now only processes the projectName.scss file. This makes it hard to generate multiple CSS files from the SASS partials.

Instead, the build should process all SASS files whose name does not start with an underscore. Projects can than easily define multiple top-level SASS files if needed. The file names of partials always starts with an underscore, so these will never be processed individually.