allow customization of the noweb reference syntax

Thanks to Sean O'Halpin for suggesting this change.

* lisp/ob-tangle.el (org-babel-tangle-clean): Use the customizable
  noweb wrappers.
* lisp/ob.el (org-babel-noweb-wrap-start): Begin a noweb reference.
  (org-babel-noweb-wrap-end): End a noweb reference.
  (org-babel-noweb-wrap): Apply the customizable noweb wrappers.
  (org-babel-expand-noweb-references): Use the customizable noweb
  wrappers.

lisp/ob-tangle.el

@@ -281,7 +281,7 @@ references."
   (interactive)
   (goto-char (point-min))
   (while (or (re-search-forward "\\[\\[file:.*\\]\\[.*\\]\\]" nil t)
-             (re-search-forward "<<[^[:space:]]*>>" nil t))
+             (re-search-forward (org-babel-noweb-wrap "[^[:space:]]*") nil t))
     (delete-region (save-excursion (beginning-of-line 1) (point))
                    (save-excursion (end-of-line 1) (forward-char 1) (point)))))
 

lisp/ob.el

@@ -120,6 +120,21 @@ be used."
   :group 'org-babel
   :type 'string)
 
+(defcustom org-babel-noweb-wrap-start "<<"
+  "String used to begin a noweb reference in a code block.
+See also `org-babel-noweb-wrap-end'."
+  :group 'org-babel
+  :type 'string)
+
+(defcustom org-babel-noweb-wrap-end ">>"
+  "String used to end a noweb reference in a code block.
+See also `org-babel-noweb-wrap-start'."
+  :group 'org-babel
+  :type 'string)
+
+(defun org-babel-noweb-wrap (regexp)
+  (concat org-babel-noweb-wrap-start regexp org-babel-noweb-wrap-end))
+
 (defvar org-babel-src-name-regexp
   "^[ \t]*#\\+name:[ \t]*"
   "Regular expression used to match a source name line.")
@@ -2163,7 +2178,8 @@ block but are passed literally to the \"example-block\"."
       (with-temp-buffer
         (insert body) (goto-char (point-min))
         (setq index (point))
-        (while (and (re-search-forward "<<\\([^ \t].+?[^ \t]\\|[^ \t]\\)>>"
+        (while (and (re-search-forward (org-babel-noweb-wrap
+					"\\([^ \t].+?[^ \t]\\|[^ \t]\\)")
 				       nil t))
           (save-match-data (setf source-name (match-string 1)))
           (save-match-data (setq evaluate (string-match "\(.*\)" source-name)))
@@ -2234,7 +2250,7 @@ block but are passed literally to the \"example-block\"."
 		  ;; possibly raise an error if named block doesn't exist
 		  (if (member lang org-babel-noweb-error-langs)
 		      (error "%s" (concat
-				   "<<" source-name ">> "
+				   (org-babel-noweb-wrap source-name)
 				   "could not be resolved (see "
 				   "`org-babel-noweb-error-langs')"))
 		    "")))