Enhancement of ExistingFileValidator, ExistingDirectoryValidator, ExistingPathValidator with additional checks

[chri50]

Hi,

appreciate if the default validators are able to check for empty,non-empty files, directories or symbolic links. e.g.

cli11.add_option("--file", file, "filename")->required()->check(CLI::ExistingFile,CLI::NotEmpty); or cli11.add_option("--file", file, "filename")->required()->check(CLI::NonEmptyExistingFile);

thx.