#18: Added WriteToTempfile() which will create a temporary output file

- We have two versions of this method:
  - One for Go versions below 1.17 using `ioutil.TempFile()`
  - One for Go versions >= 1.17 using `os.CreateTemp()`

msg_totmpfile.go

@@ -0,0 +1,20 @@
+//go:build go1.17
+// +build go1.17
+
+package mail
+
+import (
+	"fmt"
+	"os"
+)
+
+// WriteToTempfile will create a temporary file and output the Msg to this file
+// The method will return the filename of the temporary file
+func (m *Msg) WriteToTempfile() (string, error) {
+	f, err := os.CreateTemp("", "go-mail_*.eml")
+	if err != nil {
+		return "", fmt.Errorf("failed to create output file: %w", err)
+	}
+	defer func() { _ = f.Close() }()
+	return f.Name(), m.WriteToFile(f.Name())
+}

msg_totmpfile_116.go

@@ -0,0 +1,20 @@
+//go:build !go1.17
+// +build !go1.17
+
+package mail
+
+import (
+	"fmt"
+	"io/ioutil"
+)
+
+// WriteToTempfile will create a temporary file and output the Msg to this file
+// The method will return the filename of the temporary file
+func (m *Msg) WriteToTempfile() (string, error) {
+	f, err := ioutil.TempFile("", "go-mail_*.eml")
+	if err != nil {
+		return "", fmt.Errorf("failed to create output file: %w", err)
+	}
+	defer func() { _ = f.Close() }()
+	return f.Name(), m.WriteToFile(f.Name())
+}