print_help
print an usage message from an OptionParser object,
usually called by parse_args
when it encounters a help option.print_help(object)
OptionParser
instance.print_help
uses the cat
function to print out a usage message. It returns invisible(NULL)
.optparse
library, which this package is based on,
is described here: OptionParser
{parse_args}
{OptionParser}