Fix issue that sometime the element doesn't show as inViewport

#5

Thanks @tinder-salinawu

Performance enhacement

Avoid children re-render by passing static function instead of creating new function on each render #4

Upgrade IntersectionObserver polyfill

https://github.com/w3c/IntersectionObserver/tree/master/polyfill

Set inViewport to false when element is no longer in viewport

Thanks @Zeikko #1

The PR allows component to be aware of the change of in/out viewport.