Add a metadata page for inspecting configuration.

This is another metadata page that uses the new configuration system to
show all the available options, their current values, and their defaults.
This is still unstyled, but the info is there. I still need to add on a
method for extensions to register their configuration with the global
config so they show up (stuff like blog) but this is a start.

middleman-core/lib/middleman-core/application.rb

@@ -142,7 +142,7 @@ module Middleman
       cache.clear
 
       # Setup the default values from calls to set before initialization
-      self.class.superclass.config.to_h.each { |k,v| self.class.config.define_setting(k,v) }
+      self.class.config.load_settings(self.class.superclass.config.all_settings)
 
       # Evaluate a passed block if given
       instance_exec(&block) if block_given?