The `App` struct supports a "default" service which will be called when there
are no other matching services available. I propose that we support adding this
default service from the `ServiceConfig` struct, which will enable it to be used
from `App::configure`. Currently if you want to test a default service (if
you're creating a proxy for example) you must re-specify this default service
each time you construct your app. This change will allow users to put their
default services into the shared `App::configure` helper.