simplify comment inclusion

layouts/comments.erb

@@ -1,5 +1,5 @@
 <!-- comments -->
-<% unless @item[:no_comments] or @item[:is_category] %>
+<% if @item.comments? or @site.comments? %>
   <div id="disqus">
     <div id="disqus_thread"></div>
     <script type="text/javascript">

lib/default.rb

@@ -46,6 +46,10 @@ class Nanoc::Item
   def root?
     self.identifier == "/"
   end
+
+  def comments?
+    !(self[:no_comments] or self[:is_category])
+  end
 end
 
 class Category
@@ -165,6 +169,10 @@ class Nanoc::Site
       items[index + 1]
     end
   end
+
+  def comments?
+    !self["disqus_site"].nil?
+  end
 end
 
 def category name_or_cat