org-babel: allow header arg values to be lisp forms.

This allows e.g.

:file (format "%s/images/pca-scatter.png" dir)

contrib/babel/lisp/org-babel.el

@@ -462,7 +462,8 @@ may be specified in the properties of the current outline entry."
 	     (lambda (arg)
 	       (if (string-match "\\([^ \f\t\n\r\v]+\\)[ \f\t\n\r\v]+\\([^ \f\t\n\r\v]+.*\\)" arg)
 		   (cons (intern (concat ":" (match-string 1 arg)))
-			 (org-babel-chomp (match-string 2 arg)))
+                         (let ((raw (org-babel-chomp (match-string 2 arg))))
+                           (if (org-babel-number-p raw) raw (eval (org-babel-read raw)))))
 		 (cons (intern (concat ":" arg)) nil)))
 	     (split-string (concat " " arg-string) "[ \f\t\n\r\v]+:" t)))))