Feature Request: Additional "targetingOnly" Boolean argument in "Observable.fromEvent" and "Observable.fromEvents".

The "targetingOnly" argument stops the event from bubbling if it's observed during the targeting phase.

Take the following example (assume "this" is the Stage root instance):

	const child:DisplayObject = addChild(new Sprite());
	const childObs:IObservable = Observable.fromEvent(child, MouseEvent.MOUSE_DOWN);
	const stageObs:IObservable = Observable.fromEvent(this, MouseEvent.MOUSE_DOWN);
	
	childObs.subscribe(function(me:MouseEvent):void {
		trace('child down');
	});
	stageObs.subscribe(function(me:MouseEvent):void {
		trace('stage down');
	});
	
	child.dispatchEvent(new MouseEvent(MouseEvent.MOUSE_DOWN));

When the event is dispatched on the `child` instance, `stageObs` processes the event in its bubbling phase.

I could construct `childObs` to stop the propagation of all its events:

	const childObs:IObservable = Observable.
								 fromEvent(child, MouseEvent.MOUSE_DOWN).
								 peek(function(me:MouseEvent):void {
									if(me.eventPhase == EventPhase.AT_TARGET) me.stopPropagation();
								 });

or I could construct `stageObs` to filter out events it observes that aren't at the targeting phase:

	const stageObs:IObservable = Observable.
								 fromEvent(this, MouseEvent.MOUSE_DOWN).
								 filter(function(me:MouseEvent):Boolean {
								 	return me.eventPhase == EventPhase.AT_TARGET;
								 });

Either solution requires extra coding which becomes tedious if you have many parents that need to be shielded from the bubbling phase. If this were C#, I'd write an extension method. Since it isn't, this seems the least invasive place to add this feature.

Alternatively, I could see adding a `cancelBubbling()` function on `IObservable`, but since it's `Event`-specific and not necessarily related to `Observables`, I went with this instead.