Create method to extend person construct

person.go

@@ -1,6 +1,7 @@
 package atomfeed
 
 import (
+	"encoding/xml"
 	"errors"
 	"fmt"
 )
@@ -17,6 +18,10 @@ func NewPerson(name string) *Person {
 	return &Person{Name: name}
 }
 
+func (p *Person) AddExtension(name string, value any) {
+	p.Extensions = append(p.Extensions, &ExtensionElement{XMLName: xml.Name{Local: name}, Value: value})
+}
+
 func (p *Person) Check() error {
 	if p.Name == "" {
 		return errors.New("name element of person element empty")