EXPORT: Fix the options parser

* lisp/org-exp.el (org-export-add-options-to-plist): Fix the option parser

This fixes up commit aa6dba8a74016587755c250bb8cc4743a4082ea1.
Instead of looking for the start of a word (which does not work for
the options that are characters), it looks for the beginning of the
line or a whitespace character preceding the option.

lisp/org-exp.el

@@ -858,7 +858,8 @@ security risks."
       (let ((op org-export-plist-vars))
 	(while (setq o (pop op))
 	  (if (and (nth 1 o)
-		   (string-match (concat "\\<" (regexp-quote (nth 1 o))
+		   (string-match (concat "\\(\\`\\|[ \t]\\)"
+					 (regexp-quote (nth 1 o))
 					 ":\\(([^)\n]+)\\|[^ \t\n\r;,.]*\\)")
 				 options))
 	      (setq p (plist-put p (car o)