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/context.rb

@@ -13,7 +13,7 @@ module Liquid
   #   context['bob']  #=> nil  class Context
   class Context
     attr_reader :scopes, :errors, :registers, :environments, :resource_limits
-    attr_accessor :exception_handler, :template_name, :partial
+    attr_accessor :exception_handler, :template_name, :partial, :global_filter
 
     def initialize(environments = {}, outer_scope = {}, registers = {}, rethrow_errors = false, resource_limits = nil)
       @environments     = [environments].flatten
@@ -32,6 +32,7 @@ module Liquid
 
       @interrupts = []
       @filters = []
+      @global_filter = nil
     end
 
     def warnings
@@ -52,6 +53,10 @@ module Liquid
       @strainer = nil
     end
 
+    def apply_global_filter(obj)
+      global_filter.nil? ? obj : global_filter.call(obj)
+    end
+
     # are there any not handled interrupts?
     def interrupt?
       !@interrupts.empty?