feat req: lazyload recalls the last rendered component height

Hey, thank you for your time and effort for this library.

I would like to implement a feature in which react-lazyload remembers the last rendered height when the component is loaded.

At the moment `unmountIfInvisible` works well, but it messes up the scrolling in my app. Considering that the component has already been rendered during the unmount, its quite trivial to implement a `ref` and record the components rendered height that can be used for the placeholder height.

What are your thoughts regarding this?

Kind regards,
osphost