title: Cloud Firestore deep get with subcollection
body: <p>Let's say we have a root collection named 'todos'.</p>
  
  <p>Every document in this collection has:</p>
  
  <ol>
  <li><code>title</code>: String</li>
  <li>subcollection named <code>todo_items</code></li>
  </ol>
  
  <p>Every document in the subcollection <code>todo_items</code> has</p>
  
  <ol>
  <li><code>title</code>: String</li>
  <li><code>completed</code>: Boolean</li>
  </ol>
  
  <p>I know that querying in Cloud Firestore is shallow by default, which is great, but is there a way to query the <code>todos</code> and get results that include the subcollection <code>todo_items</code> automatically?</p>
  
  <p>In other words, how do I make the following query include the <code>todo_items</code> subcollection?</p>
  
  <pre><code>db.collection('todos').onSnapshot((snapshot) =&gt; {
    snapshot.docChanges.forEach((change) =&gt; {
      // ...
    });
  });
  </code></pre>