feat(router): added vetoable preLeave event

BREAKING CHANGE

Previously, vetoing was allowed on leave (RouteLeaveEvent) which caused
issues because routes had no way to recover from other route vetoing a leave
event.

Now, similar to preEnter and enter, leave event was split into vetoable
preLeave (RoutePreLeaveEvent) and non-vetoable leave (RouteLeaveEvent).

    views.configure({
      'foo': ngRoute(
          path: '/foo',
          preLeave: (RoutePreLeaveEvent e) {
            e.allowLeave(new Future.value(false));
          })
    });
chore(release): v0.12.0 (sprightly-argentinosaurus)