Summary:Add ability to specify custom left, right components, and title component. Style the `NavigationBar` according to the Platform.
Refer https://github.com/ericvicenti/navigation-rfc/pull/21
cc ericvicenti
Closes https://github.com/facebook/react-native/pull/5971
Differential Revision: D3080601
Pulled By: ericvicenti
fb-gh-sync-id: 7b921cd36b4c2ec1edf6f52629f1f9890d272dfd
shipit-source-id: 7b921cd36b4c2ec1edf6f52629f1f9890d272dfd