add withNavigationFocus HOC (#3512)

* add withNavigationFocus HOC

See:
- https://github.com/react-navigation/react-navigation/issues/51#issuecomment-276003658
- https://github.com/react-navigation/react-navigation/issues/51#issuecomment-278761705
- https://github.com/react-navigation/react-navigation/pull/3345#issuecomment-360260067

* typos

* remove unused import

* Add withNavigationFocus export

* add example TabsWithNavigationFocus

* add example TabsWithNavigationFocus

* withNavigationFocus: get navigation from context or props

* subs => subscriptions

* fix flow issues

src/react-navigation.js

@@ -88,4 +88,7 @@ module.exports = {
   get withNavigation() {
     return require('./views/withNavigation').default;
   },
+  get withNavigationFocus() {
+    return require('./views/withNavigationFocus').default;
+  },
 };