Implement custom tabs

[cheeming]

• inspired by https://github.com/react-community/react-navigation/blob/master/examples/NavigationPlayground/js/CustomTabs.js

[cheeming]

Branch here: https://github.com/Infinite-Code/SampleMobileApp/tree/2-custom-tabs

Commit here: 542f4af