title: How to define props for react-navigation with Flow
body: <p>I am working on a simple React Native App, and have decided to user react-navigation.  I have also decided to go with Flow for static type checking.  What I can't figure out, is how to define navigation related props with Flow.</p>
  
  <p>e.g. I define my App.js to use StackNavigator like so:</p>
  
  <pre><code>import StackNavigator from 'react-navigation';
  import Main from './app/containers/Main';
  
  const App = StackNavigator({
    Main: { screen: Main },
  });
  
  export default App;
  </code></pre>
  
  <p>then I define my Main class, but I don't know how to reference react-navigation in my props:</p>
  
  <pre><code>// @flow
  
  import React, { Component } from 'react';
  import { View, Text } from 'react-native';
  
  type Props = {
    navigate: ????
  };
  
  type State = {};
  
  class Main extends Component&lt;Props, State&gt; {
     ...
  }
  </code></pre>