Support for TwigBridge templates

I don't know if this is something to do with the way our project is coded, so apologies if this not actually an issue with Mailgun, but out of the box it does not support TwigBridge templates.

I'm not sure if this is the "preferred" way to do it, but it works on my project!

I'll welcome comments, corrections or criticisms on what I may be doing wrong, this is my first ever pull request so please go easy on me! :)

James.