add draft? method to identify if Post is a draft

lib/jekyll/post.rb

@@ -24,6 +24,7 @@ module Jekyll
       content
       excerpt
       excerpt_separator
+      draft?
     ]
 
     # Post name validator. Post filenames must be like:
@@ -305,6 +306,11 @@ module Jekyll
       end
     end
 
+    # Returns if this Post is a Draft
+    def draft?
+      is_a?(Jekyll::Draft)
+    end
+
     protected
 
     def extract_excerpt