Profile liquid rendering

Add a simple profiling system to liquid rendering. Each
liquid tag ({{ }} and {% %}) is processed through this profiling,
keeping track of the partial name (in the case of {% include %}), line
number, and the time it took to render the tag. In the case of {%
include %}, the profiler keeps track of the name of the partial and
properly links back tag rendering to the partial and line number for
easy lookup and dive down. With this, it's now possible to track down
exactly how long each tag takes to render.

These hooks get installed and uninstalled on an as-need basis so by
default there is no impact on the overall liquid execution speed.

lib/liquid/block.rb

@@ -33,6 +33,7 @@ module Liquid
               # fetch the tag from registered blocks
               if tag = Template.tags[$1]
                 new_tag = tag.parse($1, $2, tokens, @options)
+                new_tag.line_number = token.line_number if token.is_a?(Token)
                 @blank &&= new_tag.blank?
                 @nodelist << new_tag
                 @children << new_tag
@@ -46,6 +47,7 @@ module Liquid
             end
           when token.start_with?(VARSTART)
             new_var = create_variable(token)
+            new_var.line_number = token.line_number if token.is_a?(Token)
             @nodelist << new_var
             @children << new_var
             @blank = false
@@ -134,12 +136,8 @@ module Liquid
             break
           end
 
-          token_output = (token.respond_to?(:render) ? token.render(context) : token)
-          context.increment_used_resources(:render_length_current, token_output)
-          if context.resource_limits_reached?
-            context.resource_limits[:reached] = true
-            raise MemoryError.new("Memory limits exceeded".freeze)
-          end
+          token_output = render_token(token, context)
+
           unless token.is_a?(Block) && token.blank?
             output << token_output
           end
@@ -152,5 +150,15 @@ module Liquid
 
       output.join
     end
+
+    def render_token(token, context)
+      token_output = (token.respond_to?(:render) ? token.render(context) : token)
+      context.increment_used_resources(:render_length_current, token_output)
+      if context.resource_limits_reached?
+        context.resource_limits[:reached] = true
+        raise MemoryError.new("Memory limits exceeded".freeze)
+      end
+      token_output
+    end
   end
 end