fix(OverlayView): add position to propTypes
fix(OverlayView): add position to propTypes

* fixes #99 by @simon-engledew

RishabhJain96/react-google-maps,joshcarr/react-google-maps,RishabhJain96/react-google-maps,tomchentw/react-google-maps,ksavenkov/react-google-maps,jossi87/react-google-maps,jossi87/react-google-maps