ability to add subroutes to already registered routes

[piotrek-horodenski]

What problem does this feature solve?

This feature solves lazy loading problems in big applications, where we don't even want all routes to be registered for unauthorized users.

What does the proposed API look like?

It could be based on named routes - only named route could have its children list extended.
router.addRoutes when called with 2 arguments (name, route[]), would seek for named route by name and add route[] to its children

[posva]

duplicate of #1156

[piotrek-horodenski]

it's a duplicate of #2064