Maybe allow renderBackButton in the navigationBar config

[chirag04]

renderBackButton would let us customize the back button UI and then i can just pass that to defaultRouteConfig.

Usually an app would have mulitple StackNavigations so i would still need to add renderBackButton to all the stacknavigators defaultRouteConfigs.

One can also override the BackButton component globally like this:

import { withNavigation, NavigationBar } from '@exponent/ex-navigation';

const myBackButton = () => <View />;

NavigationBar.BackButton = BackButton;

It's a trade off and idk what's the best api here. cc @brentvatne @skevy @ide

[ide]

Globally overriding it on NavigationBar doesn't seem right to me. Better to do it per-instance for when you have multiple nav bars or to be able to create a new class that composes with NavBar and uses your BackButton. Wanting to configure all back buttons within a stack seems reasonable though.