addTo methode on Subscriptions

[mio991]

To improve code readability, I would like to have a method on Subscriptions to add this Subscription to another one.

In the end I want to be able to write:

const componentLifetime = new Subscription();

makeSomeObservable()
  .subscribe(data => doSomething(data))
  .addTo(componentLifetime)

The Implementation would be a simple:

class Subscription{
  // SNIP

  public addTo(other: Subscription): void {
    other.add(this);
  }
}