Allow custom optional visibility observer

[GeKorm]

What:
This PR allows users to

1. Omit lazy-loading completely, no need for additional dependencies
2. Provide any visibility observer

Why:
The main thing this PR achieves isn't much different than #51 I would say the only differences are:

1. It's easier to omit the observer (no need to provide a mock for onEnter to make all images visible)
2. It provides a few additional ways of defining an observer, especially because it seems many people like the child render prop pattern
3. It fixes a potential bug in onEnter

How:
Depending on what the user provides, in order:

• props.observer -> same as Add ability to use react-intersection-observer instead of react-waypoint #51
• props.children as a function -> same as props.observer
• props.children as a React element -> element is cloned and extra props onEnter and onLeave are added
• none of the above -> render the image without lazy loading (onEnter as soon as possible)

Checklist:

• Documentation
• Tests | Nope, wanted to discuss first
• Ready to be merged
• Added myself to contributors table

Please feel free to check the published fork npm install @gekorm/react-ideal-image

[GeKorm]

It's important for this to be callback otherwise this.load could have stale state (I encountered this with url being undefined)

[GeKorm]

I started working on this PR mainly because I needed to provide scrollableAncestor="window". Lazy-loading didn't work at all without this. I'm guessing it's either due to SSR or overflow: auto in a parent, both common scenarios.

[stereobooster]

It is not because of SSR (SSR should be properly handled)

[stereobooster]

I don't want to overcomplicate already complex component, rather I would search a way to separate existing component into compassable subcomponents to be able to construct out of it what you want. For example, you do not observer - simply do not use observer subcomponent. This was my initial idea, but it was hard and I decided to construct one big component for the demo and handle this idea latter.

Maybe try approach similar to state-containers.