title: Is a StackNavigator inside TabNavigator possible?
body: <p>I have created a custom tab navigator using createMaterialTopTabNavigator but inside one of the tabs I would like to create a StackNavigator. Is this possible?</p>
  
  <p>I have found lots of examples of Tabs nested inside a Stack or Drawer but not an example of a Stack inside a Tab! </p>
  
  <p>In essence I want to have a few buttons on one of the Tab Screens that navigate to some other screens but I don't want to navigate out of the Tab. (the initial Tab is always visible/selected)- just that you can click on a button and go to another screen and then back.</p>
  
  <pre>
  TabNavigator: 
   - Settings Screen (Tab 1)
         -About Us (Button when clicked opens up the About Us Screen)
         -Account Settings (Button when clicked opens up the About Us Screen)
         -Contact Us (Button when clicked opens up the About Us Screen)
   - Search Screen (Tab 2)
   - Profile Screen (Tab 3)
  </pre>
  
  <p>Any suggestions on if they is possible would be greatly appreciated! :)</p>