title: Modifying back button with react-navigation on specific screen
body: <p>Here I'm using <code>react-navigation</code> as my navigation library.</p>
  <p>How can I change the <code>back</code> button (that is added automatically by <code>react-navigation</code>) functionality for a specific screen?</p>
  <p>I mean instead of going one step in the stack of screens I want it to go 2 steps back in stack or do it manually by giving the screen name (again only on one component).</p>