Allow tag classes to be reloaded when using Liquid.cache_classes is false

Because Liquid keeps a reference to tag classes, Rails class reloading may
cause problems with custom tags. This commit introduces a setting that
allows these classes to be resolved when required.

lib/liquid.rb

@@ -38,6 +38,9 @@ module Liquid
   PartialTemplateParser       = /#{TagStart}.*?#{TagEnd}|#{VariableStart}.*?#{VariableIncompleteEnd}/om
   TemplateParser              = /(#{PartialTemplateParser}|#{AnyStartingTag})/om
   VariableParser              = /\[[^\]]+\]|#{VariableSegment}+\??/o
+
+  singleton_class.send(:attr_accessor, :cache_classes)
+  self.cache_classes = true
 end
 
 require "liquid/version"