Correctly escape strings if needed and check for it

plainText.go

@@ -15,6 +15,10 @@ func (p *PlainText) Check() error {
 		return errors.New("type attribute of plain text must be text or html if not omitted")
 	}
 
+	if p.Type == "html" && !isCorrectlyEscaped(p.Text) {
+		return errors.New("text element of plain text not correctly escaped")
+	}
+
 	if p.Text == "" {
 		return errors.New("text element of plain text empty")
 	}