[RFC] Support a fetcher that returns Observable

[leebyron]

This supports the fetcher returning an Observable, which will let us begin to experiment with support for GraphQL subscriptions and observable "live queries".

To test this, I added the crappiest possible Observable implementation to example/index.html:

if (graphQLParams.query.indexOf('subscription') === 0) {
  // If the operation smells like a subscription (#clowntown) then return an "Observable".
  return {
    // The subscribe function accepts an "Observer" which has next, error, and complete functions.
    subscribe(observer) {
      try {
        var i = 0;
        var interval = setInterval(() => {
          if (i > 10) {
            clearInterval(interval);
            observer.complete && observer.complete();
          } else {
            observer.next && observer.next({ data: i++ });
          }
        }, 500);
      } catch (e) {
        clearInterval(interval);
        observer.error && observer.error(e);
      }

      // Return a "Subscription" object with an unsubscribe function.
      return {
        unsubscribe() {
          clearInterval(interval);
        }
      };
    }
  };
}

Read more about Observable