Add Observable API

[foolip]

No description provided.

[foolip]

This description is directly from the spec, because I don't understand the API well enough to describe it myself.

@domfarolino how would you describe this to a web developer in more objective terms? In particular I think we shouldn't claim it's ergonomic here, that's up to developers to judge I think.

I'm thinking something like:

The Observable API allows you to subscribe to events in a new way. The when() method does something that is different than addEventListener().

Help? :)

[captainbrosset]

Maybe mention the when method earlier in the description.

Suggested change

	description: The `Observable` API provides a composable, ergonomic way of handling an asynchronous stream of events.
	description: The `when()` method of an `EventTarget` object, such as a DOM element, returns an `Observable` API instance which allows to ...

[domfarolino]

Developers telling us it was ergonomic¹ is what led me to ship the API and write the spec, so I feel like that description isn't a declaration as much as it is a response. But something like:

An Observable is an object representing a stream of events that you can operate on declaratively, even before it emits any events. `EventTarget#when()` returns an Observable that you can immediately subscribe to and direct with native operators, providing an ergonomic, composable alternative to `addEventListener()`.

Maybe something like that?

Footnotes

1. In a way that Promise ergonomics compare to callbacks. ↩