title: Icons not shown in React Navigation DrwaerNavigator
body: <p>I'm trying to add icons to the below simple <a href="https://reactnavigation.org/docs/en/drawer-based-navigation.html" rel="nofollow noreferrer">React Navigation DrawerNavigator</a>:</p>
  
  <pre><code>  export default createAppContainer(createDrawerNavigator({
    Home: {
      screen: HomeScreen,
      icon: 'home'
    },
    Screen1: {
      screen: Screen1
    },
    Screen2: {
      screen: Screen2
    },
    Screen3: {
      screen: Screen3
    },
  }));
  </code></pre>
  
  <p>The drawer is shown fine but icon isn't.</p>