title: AngularFireAuth. Create user with additional field
body: <p>Is it possible to create users in Firebase with an additional field?
  In my code, I can create them with "email", and "password".</p>
  
  <pre><code>  this.af.auth.createUser({
      name: formData.value.name,  // gives error*
      email: formData.value.email,
      password: formData.value.password
    }).then(
  </code></pre>
  
  <p>*error:</p>
  
  <p>signup.component.ts (26,9): Argument of type '{ name: any; email: any; password: any; }' is not assignable to parameter of type 'EmailPasswordCredentials'.
    Object literal may only specify known properties, and 'name' does not exist in type 'EmailPasswordCredentials'.</p>
  
  <p>I'd like to add additional fields, like "date of birth", "displayName", etc. </p>
  
  <p>Many thanks</p>