chore(demo): added link npm script

Author: AnthonyNahas

demo/package.json

@@ -18,7 +18,8 @@
     "generate:prerender": "cd dist && node prerender",
     "lint": "ng lint",
     "test": "ng test",
-    "postinstall": "npm link @firebaseui/ng-bootstrap",
+    "postinstall": "npm run link",
+    "link": "npm link @firebaseui/ng-bootstrap",
     "serve:prerender": "cd dist/browser && http-server",
     "serve:ssr": "node dist/server",
     "webpack:server": "webpack --config webpack.server.config.js --progress --colors"