Introducing Msg part deletion

This PR introduces a new struct field for the message parts: `del`

If the del flag is set to `true`, the msgWriter will ignore this part during the writing process.

Additionally, the `part` has now a `Delete` method that lets the user mark the part as deleted

This allows middleware to take further control of the Msg and is part of #107

msg.go

@@ -989,7 +989,13 @@ func (m *Msg) encodeString(s string) string {
 
 // hasAlt returns true if the Msg has more than one part
 func (m *Msg) hasAlt() bool {
-	return len(m.parts) > 1
+	c := 0
+	for _, p := range m.parts {
+		if !p.del {
+			c++
+		}
+	}
+	return c > 1
 }
 
 // hasMixed returns true if the Msg has mixed parts

msgwriter.go

@@ -102,7 +102,9 @@ func (mw *msgWriter) writeMsg(m *Msg) {
 	}
 
 	for _, p := range m.parts {
-		mw.writePart(p, m.charset)
+		if !p.del {
+			mw.writePart(p, m.charset)
+		}
 	}
 	if m.hasAlt() {
 		mw.stopMP()

part.go

@@ -16,6 +16,7 @@ type PartOption func(*Part)
 type Part struct {
 	ctype ContentType
 	enc   Encoding
+	del   bool
 	w     func(io.Writer) (int64, error)
 }
 
@@ -64,6 +65,12 @@ func (p *Part) SetWriteFunc(w func(io.Writer) (int64, error)) {
 	p.w = w
 }
 
+// Delete removes the current part from the parts list of the Msg by setting the
+// del flag to true. The msgWriter will skip it then
+func (p *Part) Delete() {
+	p.del = true
+}
+
 // WithPartEncoding overrides the default Part encoding
 func WithPartEncoding(e Encoding) PartOption {
 	return func(p *Part) {