Make it possible to redefine routes

[martinlasek]

Currently we are using a workaround by just define the same route again in Droplet+Setup.swift but with another destination.

It would be nice if you could do something like in JWTKeychain: pass in a delegate that subclasses the AdminPanel delegate so you can override functions.

In addition it would be nice to not change the direction (controller + function) but the route itself. Say a route /admin/backend/users is defined by AdminPanle. But you want the route to say /admin/backend/admins instead while at the same time keeping the destination/controller/function.