Demo app just to get idToken from Firebase Authentication with multiple providers.

1. You need to create public/firebase.js and input the necessary information.
2. npm install
3. npm start
4. Access to 'https://localhost:4000'
5. Press one of the Sign-In buttons
6. You can get idToken!

var firebaseConfig = {
  apiKey: "",
  authDomain: "",
  projectId: "",
  storageBucket: "",
  messagingSenderId: "",
  appId: "",
  measurementId: ""
};
firebase.initializeApp(firebaseConfig);