Add setup method and Setting object to configure AMS

lib/active_model/serializer.rb

@@ -1,6 +1,7 @@
 require 'active_model/array_serializer'
 require 'active_model/serializable'
 require 'active_model/serializer/associations'
+require 'active_model/serializer/settings'
 
 module ActiveModel
   class Serializer
@@ -12,6 +13,10 @@ module ActiveModel
         base._associations = []
       end
 
+      def setup
+        yield SETTINGS
+      end
+
       def serializer_for(resource)
         if resource.respond_to?(:to_ary)
           ArraySerializer