Re: [whatwg/dom] Disconnect single target instead of all (#126)

This would be very useful. Let me give you an example I run into:
Currently I'm building a web application with React and Redux. This application is able to run from both the server and the client. But since the server isn't supporting any browser APIs we use middleware. This middleware connects React components to JavaScript services (singletons) that enhance those components with the use of browser API's. This middleware is applied to the JavaScript client bundle only.

In one specific scenario we have components that dispatch a register action when `componentDidMount()` (React lifecycle API) is invoked. And whenever this action is dispatched it starts observing the particular node for mutations (`subscribeNode()`).

```
class ServiceCards {
  constructor() {
    this.subscribeNode = this.subscribeNode.bind(this);
    this.observer = new MutationObserver(mutationsToObserve);
  }

  subscribeNode(node) {
    node.addEventListener('transitionend', this.onTransitionEnd);
    this.observer.observe(node, mutationOptions);
  }

  onTransitionEnd(ev) {
    if (this.classList.contains('is-selected') && ev.propertyName === 'width') {
      this.classList.remove('is-selected');
    }
  }
}
```

Because React is in control when a component is either mounted or unmounted, disconnect doesn't mean so much to me. Also if a single component gets unmounted, I don't want the observer to disconnect. I'd rather dispatch an action that is able to unsubscribe a single node here.


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