DOM-like ObjectObserver API flavor

[Yuri Guller]

As of now Observable API behaves as following:

• create Observable from what to observe
• add to that Observable a logic of how (callback, the actual logic) further

I'd like to explore a possibility to add another, more DOM-like flavor to the API:

• create ObjectObserver with the callback - the how once
• with that one set the observed subjects (hence what) further

The second API flavor is more resonating with the APIs like MutationObserver and ResizeObserver.
It would also be a good extensibility exercise for the library to see if it's capable of such an enhancements.