Example: Add with-react-transition-group

[pozylon]

Just had fun using the brand new _app.js to create a small page transition example.

[timneutkens]

I'm going to close this in favor of this example: https://github.com/zeit/next.js/tree/canary/examples/with-next-page-transitions

But thank you very much for this early same of what's possible 👌🙌

[pozylon]

@timneutkens sure, no problem but I just wanted to point out that with-next-page-transitions uses a custom package named next-page-transitions that implements it's own animation/transition flow while this example uses the most popular react animation framework react-transition-group (once part of core) which allows much more flexibility and is well maintained.