Make NewCommonAttributes public

plainText.go

@@ -16,7 +16,7 @@ func (p *PlainText) isText() bool { return true }
 // newPlainText creates a new PlainText. It returns a *PlainText.
 func newPlainText(textType, content string) *PlainText {
 	return &PlainText{
-		CommonAttributes: newCommonAttributes(),
+		CommonAttributes: NewCommonAttributes(),
 		Type:             textType,
 		Text:             content,
 	}