title: Is there an onPress for TabNavigator tab in react-navigation?
body: <p>I am trying to have a DrawerNavigator when user taps on the last tab(for example 'More') in TabNavigator. </p>
  
  <p>How is this possible to achieve it without having a screen for that tab and then calling the DrawerNavigator in componentWillMount. </p>
  
  
  
  <pre class="lang-js prettyprint-override"><code>componentWillMount() {
     this.props.navigation.navigate("DrawerOpen") 
  }
  </code></pre>
  
  <p>It is kind of a hack which is not proper solution I think(This way 'More' screen is loaded), there has to be a better solution for that.</p>