Fix Rubocop Style/MethodMissing offense

Add self.respond_to_missing? method to Config module.

But...

Due to this rubocop issue:
rubocop/rubocop#3358
an offense is still detected.

lib/rubycritic/configuration.rb

@@ -28,10 +28,16 @@ def self.set(options = {})
       configuration.set(options)
     end
 
-    # rubocop:disable Style/MethodMissing
     def self.method_missing(method, *args, &block)
-      configuration.public_send(method, *args, &block)
+      if configuration
+        configuration.public_send(method, *args, &block)
+      else
+        super
+      end
+    end
+
+    def self.respond_to_missing?(symbol, include_all = false)
+      configuration.respond_to_missing?(symbol) || super
     end
-    # rubocop:enable Style/MethodMissing
   end
 end