print_help print an usage message from an OptionParser object, usually
called by parse_args when it encounters a help option.
print_help(object)A OptionParser instance.
print_help uses the cat function to print out a usage
message. It returns invisible(NULL).
Python's optparse library, which inspired this package,
is described here: http://docs.python.org/library/optparse.html
{parse_args} {OptionParser}