Enable configurable invoice file names?

[MeisterAdebar]

Okay, I understand. Would it be conceivable to make the generated file names configurable in the future?

Specifically, I am thinking about allowing users to define the file name format via a schema with placeholders.
For example: if a company name exists, it would be included in the file name together with the invoice number;
otherwise, the last name would be used instead (also combined with the invoice number).

This would allow file names to be flexibly adapted to different workflows and directory structures.

Originally posted by @MeisterAdebar in #149