Allow (currentfile) for capture templates.

* org-capture.el (org-capture-templates): document currentfile
for capture template.
(org-capture-templates): Allow to use currentfile for capture
templates.
(org-capture-set-target-location): Handle currentfile as a way
to setting the capture buffer.

* org.texi (Template elements): document currentfile for
capture templates.

doc/org.texi

@@ -6314,6 +6314,10 @@ Valid values are:
 @item (file "path/to/file")
 Text will be placed at the beginning or end of that file.
 
+@item (currentfile)
+Text will be placed at the beginning or end of the file 
+@code{org-capture} is called from.
+
 @item (id "id of existing org entry")
 Filing as child of this entry, or in the body of the entry.
 

lisp/org-capture.el

@@ -120,6 +120,10 @@ target       Specification of where the captured item should be placed.
              (file \"path/to/file\")
                  Text will be placed at the beginning or end of that file
 
+             (currentfile)
+                 Text will be placed at the beginning or end of the file
+                 org-capture is called from
+
              (id \"id of existing org entry\")
                  File as child of this entry, or in the body of the entry
 
@@ -266,6 +270,8 @@ calendar                |  %:type %:date"
 		   (list :tag "File"
 			 (const :format "" file)
 			 (file :tag "  File"))
+		   (list :tag "Current file"
+			 (const :format "" currentfile))
 		   (list :tag "ID"
 			 (const :format "" id)
 			 (string :tag "  ID"))
@@ -632,6 +638,12 @@ already gone.  Any prefix argument will be passed to the refile comand."
 	(set-buffer (org-capture-target-buffer (nth 1 target)))
 	(setq target-entry-p nil))
 
+       ((eq (car target) 'currentfile)
+	(if (not (and (buffer-file-name) (org-mode-p)))
+	    (error "Cannot call this capture template outside of an Org buffer")
+	  (set-buffer (org-capture-target-buffer (buffer-file-name)))
+	  (setq target-entry-p nil)))
+
        ((eq (car target) 'id)
 	(let ((loc (org-id-find (nth 1 target))))
 	  (if (not loc)