extensibility hook for language-dependent routes

this allows engines to register language-dependent routes without having to reimplement the language scope in their own `routes.rb` (cf. #196):

    Iqvoc.lang_routes << lambda do |routing|
      routing.resources :stuff
      routing.get "foo" => "foo#show"
    end

I played with a block-based approach similar to `Iqvoc.config`, but that didn't pan out.