feat(email-plugin): Expose template vars to template loader

[StampixSMO]

Closes #2242

Let me know if you'd like to pass more of the data object or if you somehow would hard-type the templateVars.

[michaelbromley]

I was wondering whether it makes sense to pass the entire data object, but on reflection I think it is a safer approach to expose just the templateVars. In any case, the developer is able to put any arbitrary data in templateVars anyway, so that should be sufficient.