title: Get difference between last two values of an RxJS Observable
body: <p>I have an observable tracking the scroll position</p>
  
  <pre><code> const scroll = Observable
    .fromEvent(document, 'scroll')
    .map(e =&gt; window.pageYOffset)
    .startWith(0)
  </code></pre>
  
  <p>I would like to have a second observable tracking the scroll delta <code>(newScroll - lastScroll)</code></p>
  
  <pre><code>const scrollDelta = scroll
    // ???
    .subscribe(delta =&gt; console.log('delta:', delta) )
  </code></pre>
  
  <p>How to implement something like this?
  I've tried with scan with no success.
  Thx</p>