title: Group by count for boolean values in collection
body: <p>If I have the follow data:</p>
  
  <pre><code>[{
    flagOne: true,
    flagTwo: true
  },
  {
    flagOne: true,
    flagTwo: false
  },
  {
    flagOne: true,
    flagTwo: false
  },
  {
    flagOne: true,
    flagTwo: true,
    flagThree: true
  }]
  </code></pre>
  
  <p>Note that the flag may not be set and should be treated as false.</p>
  
  <p>How do I get something like this aggregation result?</p>
  
  <pre><code>{
    flagOne: 4,
    flagTwo: 2,
    flagThree: 1
  }
  </code></pre>
  
  <p>Basically, I would like to know the # of documents in my collection grouped by each of the boolean flags.</p>