Made the following changes:
- added an archive file to store the invoice number, date and amount. This way, every invoice takes the next number from the last one.
- added some questions to the script, so the user chooses if he/she wants to input the info while running the script, or take them from the .xml file.
- line 299 in invoice.py calls okular to open the newly generated invoice. If you do not use okular, change it to meet your needs.