title: Mongo: count the number of word occurrences in a set of documents
body: <p>I have a set of documents in Mongo.  Say:</p>
  
  <pre><code>[
      { summary:"This is good" },
      { summary:"This is bad" },
      { summary:"Something that is neither good nor bad" }
  ]
  </code></pre>
  
  <p>I'd like to count the number of occurrences of each word (case insensitive), then sort in descending order.  The result should be something like:</p>
  
  <pre><code>[
      "is": 3,
      "bad": 2,
      "good": 2,
      "this": 2,
      "neither": 1,
      "nor": 1,
      "something": 1,
      "that": 1
  ]
  </code></pre>
  
  <p>Any idea how to do this?  Aggregation framework would be preferred, as I understand it to some degree already :)</p>