Refactor to create tags with a parse class method instead of new.

By moving parse out of the initializer, we can call super at the start of
the initializers for subclasses, and avoid the nasty allocate hack.

performance/shopify/comment_form.rb

@@ -1,15 +1,15 @@
 class CommentForm < Liquid::Block
   Syntax = /(#{Liquid::VariableSignature}+)/
 
-  def initialize(tag_name, markup, tokens)
+  def initialize(tag_name, markup, options)
+    super
+
     if markup =~ Syntax
       @variable_name = $1
       @attributes = {}
     else
       raise SyntaxError.new("Syntax Error in 'comment_form' - Valid syntax: comment_form [article]")
     end
-
-    super
   end
 
   def render(context)

performance/shopify/paginate.rb

@@ -1,7 +1,9 @@
 class Paginate < Liquid::Block
   Syntax     = /(#{Liquid::QuotedFragment})\s*(by\s*(\d+))?/
 
-  def initialize(tag_name, markup, tokens)
+  def initialize(tag_name, markup, options)
+    super
+
     @nodelist = []
 
     if markup =~ Syntax
@@ -19,8 +21,6 @@ class Paginate < Liquid::Block
     else
       raise SyntaxError.new("Syntax Error in tag 'paginate' - Valid syntax: paginate [collection] by number")
     end
-
-    super
   end
 
   def render(context)