added more options to fine tune positioning, omit/change prefix, modify QR code look and visual splitting of the printed number

[Unostot]

This is one of my first tries to contribute something, so it might not be the perfect pull request...help appreciated :)

I Added new options to customize and fine tune the printed labels:

• --prefix: Prefix in front of digits (default: ASN)
• --omit-prefix: Do not print prefix in front of digits (just use it in the generated QR code)
• --x-offset: Horizontal (X) offset in mm (default: 0)
• --y-offset: Vertical (Y) offset in mm (default: 0)
• --qr-scale-factor: Scale Factor of QR code (default: 0.9). Makes this option easier to play around until satisfied with the look.
• --qr-border-size: Size of white border around QR code (default: 4) Makes this option easier to play around until satisfied with the look.
• --split-at: visually split printed number at given digit position . Multiple timse allowed to add multiple spaces.

[vicegold]

Is this blocked by anything? Would love to use a different prefix :)

[alexanderbittner]

After reviewing the code and running this locally to try out the new arguments, this appears to work well. No idea why the CI doesn't work on this PR, as posted above.
Let's hope a manual review helps!