added code to pull out recipient variables from header

[wmsmith]

To support batch sending, the API recommends (https://documentation.mailgun.com/user_manual.html#batch-sending) that one should use recipient variables. I did not know of a cleaner way to implement their sending without modifying other parts of Django, so I added them to the extra_headers attribute on EmailMessage and added code to pop them out in _send().

If there is a better way, please let me know.

[pydanny]

I don't know of a better way, but I do have two requests:

1. Add Documentation. How do you get recipient variables into the header? I'll accept this pull request once that's done. 😄
2. Add a Tests. This isn't mandatory because I haven't yet documented how to run tests against django-mailgun.

[wmsmith]

Thanks for the prompt response! I added some instructions to the README along with a short code example. If you were looking for something different, please let me know.

Also, I agree on the test but I'm not sure how I would test against Mailgun without credentials. If you know of a way, let me know.

[pydanny]

That's a pretty awesome feature @wmsmith. Thank you so much for your contribution! 🚢