Add support to Cross Plataform

Discuss the possibility of adding cross platform support for push-notification-ios, which today only supports IOS.

More details in ["What do you dislike about React Native?"](https://github.com/react-native-community/discussions-and-proposals/issues/64#issuecomment-444897609) and [Reply from Facebook](https://github.com/react-native-community/discussions-and-proposals/issues/104)