add extra renderer paths

I use Galileo as CMS of our team and customize look and feel for needs. Galileo is cool enough to use and to customize. Of course we use `extra_*` options but it is not enough since we need to edit original templates. This `extra_renderer_paths` option could help people who want to change design but doesn't want to edit Galileo's original template.

But if this is not proper approach for Galileo, please just ignore and close this pull-request. :)