title: React PropTypes: Allow different types of PropTypes for one prop
body: <p>I have a component that receives a prop for its size. The prop can be either a string or a number ex: <code>&quot;LARGE&quot;</code> or <code>17</code>.</p>
  <p>Can I let <code>React.PropTypes</code> know that this can be either one or the other in the propTypes validation?</p>
  <p>If I don't specify the type I get a warning:</p>
  <blockquote>
  <p>prop type <code>size</code> is invalid; it must be a function, usually from
  React.PropTypes.</p>
  </blockquote>
  <pre><code>MyComponent.propTypes = {
      size: React.PropTypes
  }
  </code></pre>