Implement 'stack of routers' for MVC

This is the MVC companion to https://github.com/aspnet/Routing/pull/122

As routing flows, routes replace the route data and mutate a copy. This
allows users to make changes that dirty the data without affecting
undesired state changes.
Should we just pass in MvcOptions to ActionContext? I thought it sort of made sense to have it here since this is the type that instantiates the ModelState that the controller uses.