title: symfony2 console arguments
body: <p>I want to create a single <strong>named</strong> argument/option for symfony command. And want symfony to distinguish those 3 options:</p>
  
  <ul>
  <li><code>my:command</code>, which means something like <code>my:command --arg=null</code></li>
  <li><code>my:command --arg</code>, which means <code>my:command --arg=defalutValue</code></li>
  <li><code>my:command --arg=someValue</code>, which is fully explicit.</li>
  </ul>
  
  <p>I.e. I want two working modes for code under that command: default one and non-default with additional argument, and that arg should have default value.</p>
  
  <p>I understand, that I could create 2 args, but I'm looking for one-arg-to-rule-them-all solution.</p>
  
  <p>Is it possible to accomplish that with built-in classes or should I create custom ones? If solution is only available with custom classes, please tell me, where to start (i.e. "create subclass of ..." or "install a bundle named ..."), cause I'm not familiar with Symfony2's architecture.</p>