org-feed.el: Add config variant for current buffer

* lisp/org-feed.el (org-feed-alist): When FILE is nil the target
becomes the current buffer each time the feed update is invoked.

lisp/org-feed.el

@@ -116,7 +116,9 @@ to create inbox items in Org.  Each entry is a list with the following items:
 
 name         a custom name for this feed
 URL          the Feed URL
-file         the target Org file where entries should be listed
+file         the target Org file where entries should be listed, when
+             nil the target becomes the current buffer each time the
+             feed update is invoked
 headline     the headline under which entries should be listed
 
 Additional arguments can be given using keyword-value pairs.  Many of these
@@ -296,7 +298,7 @@ it can be a list structured like an entry in `org-feed-alist'."
   (catch 'exit
     (let ((name (car feed))
 	  (url (nth 1 feed))
-	  (file (nth 2 feed))
+	  (file (or (nth 2 feed) (buffer-file-name (current-buffer))))
 	  (headline (nth 3 feed))
 	  (filter (nth 1 (memq :filter feed)))
 	  (formatter (nth 1 (memq :formatter feed)))