Pass RouteManager additionally to router to modules

[pkarw]

What is the motivation for adding / enhancing this feature?

@filrak currently, because of Multistore and optimizations made in #3443 we don't want users to call router.addRoutes directly; we should then pass RouteManager instead of plain vue-router into module constructors - especially that we have more/less the same interface so it's not a breaking change even

related to #3443
related to #3454

[filrak]

@pkarw then we will loose a lot fo basic APIs from router. We can do this but I think we should make RouterManager a js proxy for router object and just proxy the addRoutes method. Then we can just export this proxy instead of a router everywhere. By doing this we are hiding unnecessary complexity and not confusing ppl that there is a router and RouterManager, wdyt?

[pkarw]

I like the idea with proxy; can you mock it up please?