Analytics web version tracking

[peterlauri]

We publish both Android/iOS apps and web using flutter. We have no way to identify what version of the app our web users are using. Would like to be able to do this in index.html:

firebase.analytics(
  {
     appVersion: '1.2.3.',
  }
);

Alternative to set it when configuring firebase:

    var firebaseConfig = {
     ...
     appVersion: '1.2.3',
     ...
    };