title: Flow: Create a flow type by extending another type
body: <pre><code>type someType = {
    keyOne: string,
    keyTwo: string,
  };
  
  type someOtherType = {
    keyOne: string,
    keyTwo: string,
    keyThree: string,
  };
  </code></pre>
  
  <p>Both of these types are objects that contain <code>keyOne</code> and <code>keyTwo</code>, the only difference is the latter <em>extends</em> the former with an additional key of <code>keyThree</code>.</p>
  
  <p>Rather than writing duplicated code, is it possible to build the <code>someOtherType</code> flow type by extending <code>someType</code>? In my mind, ES6 object rest/spread comes to mind, but I'm not sure how to accomplish something like this in Flow.</p>
  
  <p>Thanks!</p>