Add css-loader to example/app package.json

Adding this development dependency resolved a problems with `npm run build`

examples/app/package.json

@@ -33,6 +33,7 @@
     "es6-promise": "^4.1.0"
   },
   "devDependencies": {
+    "css-loader": "^0.28.5",
     "file-loader": "^0.10.1",
     "fs-extra": "^2.1.2",
     "glob": "^7.1.2",