Add support for mailgun variables

[belidzs]

Fixes #201

[lukencode]

Hi @belidzs and @klevisndoka I am going to find time in the next week to review all these PRs and get a new release going. Thanks heaps for your work (and sorry for the delay on my end)

[lukencode]

This is very similar to #204 . I suggested maybe a renderer is the best option (though we might have to change some core core)

[belidzs]

@lukencode not really, these variables primarily serve as metadata, so using them in rendering is only a secondary thing.

For instance if a developer sends a notification about a new invoice generated in their application they can pass the invoice ID so when the message is delivered (and mailgun calls your configured web service as a result) it also includes these variables as easily processable variables.

[stefankip]

Too bad this has bled to death