access subapp routes via <app-name>:<route-name>

From aio-libs/aiohttp-jinja2#163

> I think sub app routes should be available via `subapp:view-name`, which would require a `route_name` argument to `add_subapp`.

This would roughly mirror django routing and would make stuff like the `url()` jinja function work with subapps.

It would also make all `url_for` calls refering to subapps more succinct.

I think @asvetlov has some thoughts on implementation.