Added help_if_bare directive to print usage if command called with no options or arguments

Most of my command line utilities take subcommands or required options, and by default, I print the usage if only the bare command is executed.  ascertain extracts this pattern into a directive for simple reuse.

I used the directive name 'help_if_bare' -- I'm open to changing this if you can think of a more concise and clear manner.  I had also toyed with 'default_help' and 'help_on_no_arguments', but to be honest, at this point they're all ceasing to be meaningful, in the same way that if you say the same word over and over again, it stops sounding like a word -- I could really use a second opinion.

Thanks for providing methadone -- it has made life much better for me.  I was just about to provide pull requests for a simpler version and not needing to put main at the top, but I see that you beat me to it with v1.3.0!