title: How to unmount inactive screens in bottom tab navigator react-navigation?
body: <p>Im using react-navigation for react-native. Is there an option to make that inactive tab screens get unmounted like unmountInactiveRoutes: true in DrawerNavigator?? I cant find something like unmountInactiveRoutes for BottomTabNavigator.</p>
  
  <p>I have two stacknavigators inside a BottomTabNavigator and I want to unmount them automatically.</p>
  
  <ul>
  <li>BottomTabNavigator
  
  <ul>
  <li>Stack1
  
  <ul>
  <li>Screen  </li>
  <li>Screen</li>
  </ul></li>
  <li>Stack2
  
  <ul>
  <li>Screen</li>
  <li>Screen</li>
  </ul></li>
  </ul></li>
  </ul>