Escaped regular expressions when using post_url.

Previously, the post_url function would give error messages when the
post being listed contained special characters for use in regular
expressions.  These special characters are now escaped using
Regexp.escape.

lib/jekyll/tags/post_url.rb

@@ -14,7 +14,8 @@ module Jekyll
             "'#{name}' does not contain valid date and/or title."
         end
 
-        @name_regex = %r!^_posts/#{path}#{date}-#{slug}\.[^.]+|
+        escaped_slug = Regexp.escape(slug)
+        @name_regex = %r!^_posts/#{path}#{date}-#{escaped_slug}\.[^.]+|
           ^#{path}_posts/?#{date}-#{slug}\.[^.]+!x
       end