title: React Navigation: Applying separate transitions to scenes
body: <p>I want to apply different transitions to scenes in my StackNavigator in React Navigation, for my React Native (iOS) app.</p>
  
  <pre><code>export const AppNavigator = StackNavigator({
    Home: { screen: Home },
    NewChild: { screen: NewChild },
    Journal: { screen: Journal }
  });
  </code></pre>
  
  <p>I might want NewChild to load from the bottom up, like a modal. Or I might want it to slide in from right to left. It would great if this could be customised. From the documentation, it seems you can only set your StackNavigation to either card or modal for the whole thing.</p>