Allow version-less subcommands

[kbknapp]

Thanks to SShrike for pointing this out!

Most git-like CLIs don't implement version on a per-subcommand basis. Granted there are times when you may wish to do so. But having the option to disable the versions for all but the top level command easily may also be desired.

This issues requests adding an App::versionless_subcommands(bool) which can disable all version below the current command in the hierarchy.