title: commander.js : how to specify required cli argument
body: <p>I'm using commander.js package for parsing command-line arguments: I'd like to make a flag non-optional, the API and tests in the <a href="https://github.com/visionmedia/commander.js">git repo</a> loosely mention making a flag required, but I usually need to be hit over the head with instructions.</p>
  
  <p>Is it actually possible, and will the script throw if the requirement is not met?</p>