Apply validators to defaults if no value was set from the comand line

[Ozaq]

A rather common use case for me is to validate the existence of files passed to my program via commandline arguments. CLI11 features Opt::check(CLI::ExistingFile) but It seems to not get called if the option was not present on the command line but there is still a default, e.g.

std::filesystem::path file{"myfile"};
CLI::Option* opt1 = app.add_option("--inifile", file, "Path to your inifile");

I saw the workaround in #115 so consider this just as a though for improvement.

[phlptp]

So with #325 the value of the option specified as a default_str is run through the validators, which is wasn't previously. It still isn't going to check the value just stored in a variable that might get assigned.