Add a filter for the templates directory. 

This is a minor change, but one that I feel would be useful for theme developers. I'm currently working on a theme that integrates Crowdfunding & EDD, and needed to override the default stylesheet. Obviously there's nothing wrong with just having it in the `edd_templates` dir, but for better organization I prefer to create a separate directory called `templates`, which can house custom templates for EDD as well as other plugins I may integrate.