Add augmentScene and augmentNavigationBar methods

[jesseruder]

Call _subscribeToFocusEvents with navigator instead of this. Otherwise navigator.navigationContext crashes.

Add augmentScene and augmentNavigationBar which can be overridden to add custom components to every scene/navigation bar.

[jesseruder]

Also deleted duplicate get navigationContext()

[jadsonlourenco]

I think this is the problem that I get here #3
I will test this fork, thanks!

[ide]

Looking at this again I think a prop would be preferable:

let navigationBar = this.props.renderNavigationBar({
  routeMapper: this._routeRenderer.navigationBarRouteMapper,
  style: [ExNavigatorStyles.bar, this.props.navigationBarStyle],
});

And use defaultProps to render a basic Navigator.NavigationBar by default.

For the inheritance case I think we could override defaultProps:

class CustomNavigator extends ExNavigator {
  static defaultProps = {
    ...ExNavigator.defaultProps,
    renderNavigationBar: props => <CustomNavigationBar {...props} />,
  };
}

[jesseruder]

@jadsonlourenco this fixes the problem in #3 so you can use this branch for now. I'm going to work on some of the other things I changed in this pull so I won't merge it quite yet.

[ide]

Now I'm thinking this actually could be a prop (this.props.augmentScene). Then the owner of the ExNavigator (instead of its subclass) could write:

render() {
  return <ExNavigator augmentScene={this._augmentScene} />;
}
@autobind
_augmentScene(scene, route) {
 ...
}

[ide]

Thanks Jesse!