Allow routes to be marked as allowed slugs.

I had some issues with the marketable_urls function while using Refinery with Spree. I wanted to set some routes as excluded from the reserved words list, but not all. This change now allows for the following:

`match '/some_url', :to => 'some_controller#index', :allow_slug => true`

`some_url` will now not be in the `Refinery::Page.friendly_id_config.reserved_words` array.