title: React Navigation: Scroll to top if tab is the same as active tab
body: <p>I have a tab-based app. If I'm currently on the first tab, scrolled all the way down, and I click the tab again, I want it to scroll to the top. Is this possible?</p>
  
  <p>I know I should have a scroll view with a reference and then use <code>this._scrollView.scrollTo(0)</code>, but how can I detect when the user taps the tabbar and decide whether it is the same tab?</p>