Flag and/or option fallback function

[sasq64]

I want to parse special flags that begins with a dash and contains a single digit to mean compression levels for my zip like compression tool.

Right now I need to handle this with a special check inside a callback for positional arguments.

I would like to have something like:

opts.add_flag_fallback("-<digit>", [&](const char d) {
    compressionLevel = d - '0';
}, "Set compression level (0-9)");

[henryiii]

I'm thinking this could be a special type of add:

app.add_digit(Optional int value, "description");

[henryiii]

This is now possible!