Add support for mailgun EU-region api

[OlafHaag]

Hi! The contributing guidelines said to open an issue for discussion before sending a pull request.
I was spending hours on figuring out why my email notification with mailgun wouldn't work until I finally found this comment that suggested that the current implementation isn't working with the EU region mailgun api. I switched to EU region because of GDPR concerns.

I'd like staticman to work with the mailgun eu region api and I already tested it successfully on my own staticman instance, hence solving this issue.

[alexwaibel]

Will consider adding this once the repo is in a functional state again.