title: How to declare proptypes to validate an array of specific strings?
body: <p>A value passed to component can be an array of strings. That strings means different directions: <code>top</code>, <code>right</code>, <code>bottom</code>, <code>left</code>.</p>
  
  <p>How to specify proptypes for component to allow only array of specified directions?</p>
  
  <pre><code>  &lt;Arrows
      directions=['left', 'bottom'] // accepts the array
    /&gt;
  </code></pre>
  
  <pre><code>  &lt;Arrows
     directions=['left', 'back'] // doesn't accept the array
    /&gt;
  </code></pre>