pass function as email template

[smassin]

I have a use case where the emails need to be sent in the language of the user. Unless I'm missing something, the current implementation doesn't offer a mechanism to do so.

I've made a fork for myself that makes a more generic solution to allow you to pass a callback function(options) {} so users can be sent different email templates based on their details.

I can create a PR if such a change is desired to be integrated.