ox-koma-letter.el: Support multiple LCO files

* ox-koma-letter.el (org-koma-letter-template): Treat `#+LCO:'
  variable as a list of multiple LCO files separated by spaces.

Example:

  #+LCO: Default Signed

will result in the following LaTeX fragment

  #+BEGIN_SRC latex
  \LoadLetterOption{Default}
  \LoadLetterOption{Signed}
  #+END_SRC

Signed-off-by: Alan Schmitt <alan.schmitt@polytechnique.org>

contrib/lisp/ox-koma-letter.el

@@ -260,7 +260,12 @@ holding export options."
 	 (signature (plist-get info :signature)))
      (concat
       ;; Letter Class Option File
-      (when lco (format "\\LoadLetterOption{%s}\n" lco))
+      (when lco 
+	(let ((lco-files (split-string lco " "))
+	      (lco-def ""))
+	  (dolist (lco-file lco-files lco-def)
+	    (setq lco-def (format "%s\\LoadLetterOption{%s}\n" lco-def lco-file)))
+	  lco-def))
       ;; Define "From" data.
       (when sender (format "\\setkomavar{fromname}{%s}\n" sender))
       (when from-address (format "\\setkomavar{fromaddress}{%s}\n" from-address))