feat: add part getter

pkg/message/parser/parser.go

@@ -1,6 +1,7 @@
 package parser
 
 import (
+	"errors"
 	"io"
 	"io/ioutil"
 
@@ -30,6 +31,25 @@ func (p *Parser) NewWriter() *Writer {
 	return newWriter(p.root)
 }
 
+func (p *Parser) Header() message.Header {
+	return p.root.Header
+}
+
+func (p *Parser) Part(number []int) (part *Part, err error) {
+	part = p.root
+
+	for _, n := range number {
+		if len(part.children) < n {
+			err = errors.New("no such part")
+			return
+		}
+
+		part = part.children[n-1]
+	}
+
+	return
+}
+
 func (p *Parser) parse(r io.Reader) (err error) {
 	e, err := message.Read(r)
 	if err != nil {