Send attachments with SMTP

[LinusGeffarth]

Example usage with a pdf file from the Public folder:

let directory = DirectoryConfig.detect()
let configDir = "Public/storage/pdf"
let pdfData = try Data(contentsOf: URL(fileURLWithPath: directory.workDir)
    .appendingPathComponent(configDir, isDirectory: true)
    .appendingPathComponent(some.pdf, isDirectory: false))

let attachments: [Attachment] = [
    Attachment(data: pdfData, mime: "application/pdf", name: "some.pdf", inline: false)
]
return try req.view().render("my-template").map { (view) -> [Mailer.Message] in
    return users.map { user in
        return Mailer.Message(
            from: "from@example.com",
            to: "to@example.com",
            subject: "Example subject", text: "Example subject",
            html: String(data: view.data, encoding: .utf8) ?? "",
            attachments: attachments
        )
    }
}

Again, additions only, shouldn't break anything.

[rafiki270]

Would you mind updating the documentation please?