Add support for custom directories to read file contents

[andrewlimaza]

Currently works for paid-memberships-pro/email

Would be good to integrate with custom directories from other plugins.

Example my-plugin/emai/template.htmll

[andrewlimaza]

I have added a custom email template using the pmproet_templates filter and when I try to send an email using the PMPro email class (and using the exact same template name) it does not show what is saved in Email Templates add on that I saved for that particular template.