org-ascii.el: Change the underlining characters for headlines of level 1 and 2

* org-ascii.el (org-export-ascii-underline): Change the
default underlining characters for headlines of level 1 and 2.
Also introduce \. as the underline character for headlines of
level 5.

Thanks to Mathias Bauer for suggesting this.

lisp/org-ascii.el

@@ -36,7 +36,7 @@
   :tag "Org Export ASCII"
   :group 'org-export)
 
-(defcustom org-export-ascii-underline '(?\- ?\= ?\~ ?^ ?\# ?\$)
+(defcustom org-export-ascii-underline '(?\= ?\- ?\~ ?\^ ?\. ?\# ?\$)
   "Characters for underlining headings in ASCII export.
 In the given sequence, these characters will be used for level 1, 2, ..."
   :group 'org-export-ascii