$ionNavBackButton: allow for icons other than ionicons

The ionNavBackButton directive’s code expects an icon with an “ion-” class otherwise it includes one automagically.

What would be the correct way to extend/modify that directive to allow for i.e. “fa-” classes or to disable the auto-icon functionality, so that we could use other icons (FontAwesome, icomoon, etc)?

I was also thinking on using $ionicNavBarDelegate on a vanilla button, but that would mean I would have to replicate all of the ionic back button functionality/animations in my new button, am I correct?