title: Can I use specific token for firebase?
body: <p>I am developing a React Native app and I using Firebase push notification service.
  I have users and all users have their own user token, I want to use these user tokens for send push notification.</p>
  <p>First I thought, when user login to app I can register the user token to topic but I'm not sure about that too.</p>
  <p>So is there any way to use custom token for send push notification to specific user?</p>