Disallow periods in slugs. Fixes Issue #10.

src/content.lisp

@@ -77,7 +77,7 @@ bound to the current subclass."
   (or (char<= #\0 char #\9)
       (char<= #\a char #\z)
       (char<= #\A char #\Z)
-      (member char '(#\_ #\- #\.))))
+      (member char '(#\_ #\-))))
 
 (defun slugify (string)
   "Return a version of STRING suitable for use as a URL."