Different route animation behaviour depending on from/to routes

[stefanholzapfel]

I needed different behaviour for my route animations, depending on the route the app is going to. E.g. components should leave to the left when going deeper in the routing tree, leaving right when going "back". The simplest way was to enable the router to accept a factory function for the "animate" property that would be evaluated on every route change.

See pull request: #646