Attachments using readers

[mcuadros]

This PRs adds an alternative method of attach or embed files in Message using a io.Reader allowing to attach files made on the fly and without needing to save a file to disk.

[alexcesaro]

Thank you but it is already possible using SetCopyFunc:

var r io.Reader // Your reader
m.Attach("foo.txt", gomail.SetCopyFunc(func(w io.Writer) error {
    _, err := io.Copy(w, r)
    return err
}))

[mcuadros]

Oh great! Thanks