Fix modifiers inheritance issue

There seems to be loading issues with the `inherited` hook. This has
been reported as not being an issue with this commit's code.

Author: davidalejandroaguilar

lib/phlexy_ui/base.rb

@@ -8,15 +8,18 @@ def initialize(*base_modifiers, **options)
     end
 
     class << self
-      attr_reader :modifiers
-
-      private
+      def modifiers
+        @modifiers ||= {}
 
-      def inherited(subclass)
-        super
-        subclass.instance_variable_set(:@modifiers, (@modifiers || {}).dup)
+        if superclass.respond_to?(:modifiers, true)
+          superclass.send(:modifiers).merge(@modifiers)
+        else
+          @modifiers
+        end
       end
 
+      private
+
       def register_modifiers(modifiers)
         @modifiers ||= {}
         @modifiers.merge!(modifiers)