fix: add custom sender name to "from" header

[kangmingtay]

Current workaround:

mail := gomail.NewMessage()
from := mail.FormatAddress(address, senderName)

// create mailer struct
mailer := &Mailer{..., From: from}

// invoke Mail method
err := mailer.Mail(...)

Future use:

mailer := &Mailer{..., From: "[email protected]", SenderName: "Alice"}
err := mailer.Mail()

I think it would be nicer if i didn't have to create mail and from and FormatAddress of the sender outside of the mailme package when it can be handled inside the Mail method. :)