Add icons to thumb instead of only image

[mazing]

Right now it's possible to add an image to thumb, but what about icons?

Is there a reason we can only define source for image instead of letting the user add the image as they like, enabling one to insert <Icon>, <Svg>, <Image>, etc?

[LRNZ09]

I also have this need

EDIT: somewhat related https://github.com/jeanregisser/react-native-slider/pull/134