Added FEED + FEED operator to merge two feeds.

Added FEED sorting routine.
Added FEED_ITEM.link: detachable FEED_LINK that represents the main feed link.
Comments.

library/text/parser/feed/src/rss/rss_2_feed_parser.e

@@ -112,7 +112,7 @@ feature -- Access
 										else
 											e.set_content (xml_element_code (x_content), Void)
 										end
-										Result.add_item (e)
+										Result.extend (e)
 									end
 								end
 							end