title: Mongo aggregate $slice for subdocs
body: <p>Given docs like this:</p>
  
  <pre><code>var user = { 
    _id: '53176dc214d1f0d0ec42ae54',
    friends: [ '53176dc214d1f0d0ec42ae54', '5317766614d1f0d0ec42ae5e' ]
  };
  </code></pre>
  
  <p>I would like, in an aggregate pipeline, to return both count of each friends (as friend_total) as well as the first 10 friends ids.</p>
  
  <p>I have this so far which returns the data I want - I need the equivalent of $slice for friends though.</p>
  
  <pre><code>{
    $unwind: '$posters',
  }, {
  $group: {
    _id: '$_id',
    posters:{$push:'$posters'},
    sum: {
      $sum: 1
    }
  }
  </code></pre>