Improve Mail templates infrastructure

Currently we have a lot of duplication in mail templates as they are processed only as a single file or content.

In order to better manage mail templates we need to enable "include" directive to have ability to split templates into components (same as CMS include directive)

This directive allows to include another template inside the calling template thus giving the opportunity to exact common blocks like headers, footers, order details blocks etc.

Note that "include" directive must work transparently with both file and CMS sources.