parse-options hidden options

[brandon1024]

Right now, there's no way to define an option that won't be shown in the standard show_usage_with_options() output. This would be particularly useful to be able to do if you wanted to add options that were aliases for other options. For example:

USAGE("git chat import-key ((-f | --fingerprint) <id>) [--gpg-home <path>] ")
...
OPT_LONG_STRING("gpg-home", "path", "path to the gpg home directory", &gpg_home_dir),
OPT_STRING('f', "fingerprint", "id", "id of the public key to import", &fingerprint),
+ OPT_HIDDEN(0, "fpr", OPTION_STRING_T, &fingerprint),
OPT_STRING('k', "key", "path", "path to exported public key file", &key_path),
OPT_BOOL('h', "help", "show usage and exit", &show_help),
OPT_END()

This OPT_HIDDEN macro could have the following signature:
#define OPT_HIDDEN(S,L,T,V) { (S), (L), NULL, NULL, OPTION_HIDDEN_T | (T), (V) }

Since str_name and desc won't be needed, null is fine.