Include tax fields

[strzibny]

• Each item row should have a tax field
• There should be a tax info associated with the total amount
  • subtotal / taxable / tax rate / tax due / total due

[strzibny]

Done in https://github.com/strzibny/invoice_printer/commit/08c967815fc9b8fcf468386ef61e91c0dd34c55c.