Add support for Mandrill subaccounts and Mailgun routes

This PR adds a support to the newly added Mandrill subaccount features, and update the possible options.

It also adds support for Mailgun routes (which I didn't have the courage to do, but it's done now).

Can you review @juriansluiman? I'd suggest we tag this 1.1.0 as it adds quite a bunch of new features.