Specify default params in the params key in route config

[brentvatne]

export default createBottomTabNavigator(
  {
    A: {
      screen: Screen,
      params: { title: 'First One', icon: 'activity' },
    },
    B: {
      screen: Screen,
      params: { title: 'Second One', icon: 'aperture' },
    },
  },
  {
    tabBarOptions: {
      activeTintColor: '#000',
      inactiveTintColor: '#eee',
    },
  }
);

I went for the name params instead of defaultParams or initialParams because it's consistent with how we use navigationOptions in the routeConfig. I think it is clear to people that these are default params. If that doesn't turn out to be the case then we can adjust by renaming both navigationOptions and params in the route config object to something that implies default more strongly.