title: onEnter/onExit method in React Native Component (react-native-router-flux)
body: <p>So I can use the onEnter/onExit method at the root of my app in the router definitions and it works perfectly fine:</p>
  
  <pre><code>&lt;Scene key="arena" hideNavBar={true}  onEnter={() =&gt; console.log("Entered")} component={ArenaPage} /&gt;
  </code></pre>
  
  <p>Is there any way I can do this inside the component itself so that I can update the local state??</p>
  
  <pre><code>export default class ArenaPage extends Component {
      onEnter () {
          this.setState(...)
      }
      // Render blah blah blah...
  }
  </code></pre>
  
  <p>If not possible, Is there anyway to trigger componentWillUnmount when navigating away from Scene (Actions.[key])</p>