Support Multiple Domains

You can now use multiple domains to send emails with Mailgun

let mailgun = Mailgun(apiKey: "<api key>")
services.register(mailgun, as: Mailgun.self)

// Put this extension at the bottom or create a new file for it
extension Mailgun.DomainConfig {
    static var euDomain: Mailgun.DomainConfig {
        return Mailgun.DomainConfig("mg.example.com", region: .eu)
    }
    static var usDomain: Mailgun.DomainConfig {
        return Mailgun.DomainConfig("mg2.example.com", region: .us)
    }
}

mailgun.send(message, domain: .euDomain, on: req) 

The new major version for this release is 3.0.0 to align with the supported Vapor version. 4.0.0 tag coming soon.