title: How to update DrawerLabel text (via navigationOptions) on DrawerOpen/Close event
body: <p>I have define drawerLabel in navigationOptions as follows : </p>
  
  <pre><code>export default class Something extends React.Component {
  
  static navigationOptions = {
      drawerIcon: () =&gt; (
        &lt;Image source={drawerImage} style={styles.imageStyle} /&gt;
      ),
  
     drawerLabel: ()=&gt;{
  
          if(~some condition~){
                  return 'New Label'
              }else if(~some other condition~){  
                  return 'Another Label'
              }else{
                  return 'label'
              }            
      },
  };
  </code></pre>
  
  <p>This updates the label perfectly when any drawerItem is pressed in the drawer.</p>
  
  <p>(apparently opening ANY drawer item updates the navigationOptions of all routes)</p>
  
  <p><strong>But I want this drawerLabel to be updated  on drawer.Open() call as well.</strong>
  <em>or when left-header is pressed on any screen (triggers drawer.Open() )</em> </p>
  
  <p>So, how can I achieve this behaviour on DrawerOpen/Close ? 
  I am able to achieve this on pressing any drawerItem </p>