Forward template variables to the template loader

[StampixSMO]

Is your feature request related to a problem? Please describe.
We would like to customise template loading based on order data. For example, a different email for promotions.

Describe the solution you'd like
Forwarding the templateVars to the TemplateLoader would make this possible.

Describe alternatives you've considered
No alternatives possible with the current API. We could use the EmailGenerator to override the already read body, but that is currently a synchronous function and hence that would have to change too.

Additional context
I'll open a PR for this.