support more formatting with --filename-format option

[rmilecki]

Before this change --filename-format accepted string with up to three
variables only ("date", "invoice_number" and "desc"). It also couldn't
deal with "date" being a list of dates.

This change allows using any parsed field. It supports date in any
field. It also deals with list of dates by picking the first one.

Example usage:
--filename-format "{date} {issuer} {invoice_number} {currency}.pdf"

[rmilecki]

I'd like to propose this change as an alternative solution to both: https://github.com/invoice-x/invoice2data/pull/375 https://github.com/invoice-x/invoice2data/pull/384

This also fixes https://github.com/invoice-x/invoice2data/issues/374