Allow passing in a custom DatePickerIOSComponent

[codynguyen]

For example the new https://github.com/react-native-community/datetimepicker. This is mostly to fix the problem with iOS Dark Mode where the text would turn white making it invisible in the white background.

import RNDatePicker from '@react-native-community/datetimepicker';

<DatePicker
    iOSDatePickerComponent={RNDatePicker}
    textColor={variables.text_color_dark}
/>

[coveralls]

Coverage decreased (-0.8%) to 96.552% when pulling e5a8dd1 on codynguyen:ios-datepicker-component into 2a209bd on xgfe:master.