ExistingDir() still checked when program run with --version

When using `kingpin.Version("...")`, passing `--version` means `.Required()` arguments do not have to be provided. I have an argument with a `.Default()` that must be an `.ExistingDir()`. The default path does not exist on the system. When passing only `--version`, I get an error that the default directory doesn't exist, rather than the version. Passing `--version --help` means the version is printed correctly.

It would be very useful if kingpin did not check that `.ExistingDir()`s actually existed if the program is run with only the `--version` flag.