title: Can I group by multiple fields using Angular-Filter?
body: <p>I'm using <a href="https://github.com/a8m/angular-filter" rel="noreferrer">Angular-Filter</a>'s <a href="https://github.com/a8m/angular-filter#groupby" rel="noreferrer">groupBy</a> filter. </p>
  
  <p><strong>Example from GitHub</strong>:</p>
  
  <pre><code>$scope.players = [
    {name: 'Gene', team: 'alpha'},
    {name: 'George', team: 'beta'},
    {name: 'Steve', team: 'gamma'},
    {name: 'Paula', team: 'beta'},
    {name: 'Scruath', team: 'gamma'}
  ];
  
  &lt;ul&gt;
    &lt;li ng-repeat="(key, value) in players | groupBy: 'team'"&gt;
      Group name: {{ key }}
      &lt;ul&gt;
        &lt;li ng-repeat="player in value"&gt;
          player: {{ player.name }}
        &lt;/li&gt;
      &lt;/ul&gt;
    &lt;/li&gt;
  &lt;/ul&gt;
  </code></pre>
  
  <p>So, the example groups the players collection by <strong>team</strong>. Imagine there's an <strong>age</strong> property for every player object:</p>
  
  <pre><code>$scope.players = [
    {name: 'Gene', team: 'alpha', age: 19},
    {name: 'George', team: 'beta', age: 19},
    {name: 'Steve', team: 'gamma', age: 23},
    {name: 'Paula', team: 'beta', age: 23},
    {name: 'Scruath', team: 'gamma', age: 23}
  ];
  </code></pre>
  
  <p>I want to group by <strong>team</strong> and <strong>age</strong>. How can I do that?</p>