Allow optional strings in --output patterns

[ilikepeaches]

Default file names include certain constant elements only if a certain data field is populated. For instance, the default NY Times filenames only include " - " after the date string on Sundays, when the puzzle actually has a title.

NY Times - 20241207.puz
NY Times - 20241208 - Just for Fun.puz

but filename: '%prefix - %Y%m%d %a - %title' in xword-dl.yaml or -o '%prefix - %Y%m%d %a - %title' on the command line produce

NY Times - 20241207 Sat - .puz
NY Times - 20241208 Sun - Just for Fun.puz

The dividing elements preceding the title are included even when there is no title.

Could you please replicate that feature for user-defined patterns, or, if it already exists, share with me the syntax for doing so, and perhaps make that syntax easier to discover?

I would imagine a grouping system, perhaps with delimiters around portions of the pattern that should only be included if the relevant token is populated, something like -o '%prefix - %Y%m%d %a''( - %title)'

[thisisparker]

This is a good idea! I could probably simplify the way the delimiter omissions work, too. I'll look into implementing this.