Mailgun: support templates

[Colmea]

Hello,

I use Mandrill for months, but since it's not free anymore, I'm going to use Mailgun. I saw there is a template feature in Mailgun, but nothing in the SlmMail Mailgun support.

Is it a missing feature ? Or I'm missing something ?

Thanks for your awesome work !

[roelvanduijnhoven]

@aimfeld This does seem to be missing. I'll mark as enhancement.