title: React-navigation and custom left button onPress?
body: <p>I want to create a custom <code>left</code> button in <code>react-navigation</code>. I need the <code>onPress</code> event of this <code>left</code> button to go to a specific screen; how do I do this? Ideally I want to stay away from writing my own redux navigation as that got complicated. Can I do this without redux?</p>