Improvement: add unit for the VAT

[lfdmn]

Hi and thanks for the module! I'm trying to set it up

It took me readying through the code, not that it's a bad thing :D, to figure out in which format to enter the VAT. i.e for 24% VAT, should I enter 24, 0.24 or 1.24.

How about adding the unit to the form? Or change the unit to a real percentage (24%) which is most commonly used.