add global_filter

add a global filter using a proc
only add one proc and not an array
add tests to make sure the global_filter is applied after native filters

lib/liquid/template.rb

@@ -179,20 +179,15 @@ module Liquid
       when Hash
         options = args.pop
 
-        if options[:registers].is_a?(Hash)
-          registers.merge!(options[:registers])
-        end
+        registers.merge!(options[:registers]) if options[:registers].is_a?(Hash)
 
-        if options[:filters]
-          context.add_filters(options[:filters])
-        end
+        context.add_filters(options[:filters]) if options[:filters]
 
-        if options[:exception_handler]
-          context.exception_handler = options[:exception_handler]
-        end
-      when Module
-        context.add_filters(args.pop)
-      when Array
+        context.global_filter = options[:global_filter] if options[:global_filter]
+
+        context.exception_handler = options[:exception_handler] if options[:exception_handler]
+
+      when Module, Array
         context.add_filters(args.pop)
       end