Add frontend and backend routes method

Hello all !

In following of our conversation in the goal to translate engine routes. 
I submit this PR in order to add the feature of frontend and backend routes in routes.rb

Now i can do this :

```ruby 
Refinery::Core::Engine.frontend_routes do
  # Frontend routes
  namespace :inquiries, :path => '' do
    # localized do
      get Refinery::Inquiries.page_path_new, :to => 'inquiries#new', :as => 'new_inquiry'

      resources :contact, :path => Refinery::Inquiries.post_path, :only => [:create],
                :as => :inquiries, :controller => 'inquiries'

      resources :contact, :path => '', :only => [], :as => :inquiries, :controller => 'inquiries' do
        get :thank_you, :path => Refinery::Inquiries.page_path_thank_you, :on => :collection
      end
    # end
  end
end

Refinery::Core::Engine.backend_routes do
  # Admin routes
  namespace :inquiries, :path => '' do
    namespace :admin, :path => Refinery::Core.backend_route do
      resources :inquiries, :only => [:index, :show, :destroy] do
        get :spam, :on => :collection
        get :toggle_spam, :on => :member
      end

      scope :path => 'inquiries' do
        resources :settings, :only => [:edit, :update]
      end
    end
  end
end
```

I'm clearing inspired of Spree : https://github.com/spree/spree/blob/b66ea0229646062acb7b6f89ac2447f5ffc862fb/core/lib/spree/core/routes.rb

It's backward compatible.

Now, i try to find a way to add the localized method of route_translator on frontend_routes. If you know how to do it, please help us! :+1: 

https://github.com/enriclluelles/route_translator/issues/75