option for labeling invoices

[Keegil]

many of our customers need their invoices marked with specific numbers or codes in addition to the address. implement an option for such labeling and make it appear right below the adress on the invoice, just like it would if you were to send a letter. this option should be called "Fakturamerking" and it needs to be available in the specifications for each individual sale, as this might vary from sale to sale