Add ability to turn off slug scoping

-- Add configuration variable to turn off slug scoping.
-- When slug scoping is off, custom slugs are allowed to have
   slashes in them, and essentially become custom paths, allowing
   the user to customize the routes any way they want, regardless
   of page nesting. Users can still organize their pages in a tree structure
   for menus or other purposes, but this setting allows them to setup
   their own url structure. This is especially useful for migrating an
   existing site to Refinery, allowing them to keep their current urls
   in place.
-- Add a useful deep merge to Hash