Add other transitions for nav view

[John Slemmer]

I've been working on an app and thought this animation would be nice but it doesn't appear to be available.

http://ionicframework.com/docs/api/directive/navDirection/

And you wouldn't necessarily need to be constrained by the Ionic transitions. You could even have other values be settable to ionNavDirection. For instance something like https://davidwalsh.name/demo/css-flip.php

Also, I would take a crack a making it happen but that layer of UI I no experience. In playing around I was able to get something working but it just doesn't look very good and complete. Anyway, just leaving this issue here to track it for something to get done down the road..