Provide API to disallow POSIX clustered short options: this is required to implement #258 (Picocli-based groovy.CliBuilder alternative).

CliBuilder exposes a posix property that (if set to false) installs a commons-cli parser that does not allow POSIX clustered short options.

Picocli by default supports POSIX clustered short options but does not have an option to switch this off. Provide such a switch to facilitate the picocli-based CliBuilder implementation.