title: Extend Immutable.js types
body: <p>I would like to extend functionality of Map class. For example, I want to do following:</p>
  
  <pre><code>const someMap = Immutable.Map({ a: 1, b: 2 });
  const {a, b} = someMap.batchGet('a,' 'b');
  </code></pre>
  
  <p>How can I correctly add 'batchGet' method to Map class?</p>