refactor(sass): moved common config values to environment variables

Author: danbucholtz

config/sass.config.js

@@ -8,7 +8,7 @@ module.exports = {
    * from the sass build. The directory which it is saved in
    * is set within the "buildDir" config options.
    */
-  outputFilename: 'main.css',
+  outputFilename: process.env.IONIC_OUTPUT_CSS_FILE_NAME,
 
   /**
    * sourceMap: If source map should be built or not.