RFC options should be available when sending mails

[bounoable]

Users might want to change the Message-ID or Date headers of a mail. They should be able to do so like this:

dog := postdog.New()
dog.Send(
  context.TODO(),
  letter.Write(),
  postdog.RFCOptions(
    rfc.WithClock(...), // custom clock
    rfc.WithIDGenerator(...), // custom message id generator
  ),
)