Do not mutate passed-in config obj
Do not mutate passed-in config obj

Fixes #12

Khan/react-tween-state,cyhunter/react-tween-state,cyhunter/react-tween-state,mcanthony/react-tween-state,GuoYongfeng/react-tween-state,swhite24/react-tween-state,GuoYongfeng/react-tween-state,chenglou/react-tween-state