Added selectedItem to props.renderButton args.  Combined args into single object so users can use argument destructing instead of having to track all method arguments.  Added getSelectedItem helper function.  Added propType for renderButton.
Added selectedItem to props.renderButton args.  Combined args into single object so users can use argument destructing instead of having to track all method arguments.  Added getSelectedItem helper function.  Added propType for renderButton.

GeekyAnts/NativeBase,GeekyAnts/NativeBase,GeekyAnts/NativeBase,GeekyAnts/NativeBase,GeekyAnts/NativeBase