title: QCommandLineOption with optional value
body: <p>I would like to define a command-line option with an optional value. For example:</p>
  
  <pre><code>QCommandLineOption pngOption("png",
      QCoreApplication::translate("main", "Generate PNG diagrams."),
      "items", "*");
  </code></pre>
  
  <p>I'd like to use this option as follows:</p>
  
  <pre><code>myApp --png diagrams
  myApp --png
  </code></pre>
  
  <p>The second usage is --png without a value (optional value) and causes an error "missing value after --png". I would think that in that case the value of the option should be set to the defaultValue argument of the QCommandLineOption constructor, which is provided as "*". But apparently, Qt does not do this.</p>
  
  <p>So, my questions are:</p>
  
  <ul>
  <li><p>how to specify an option with an optional value (e.g., --png [items])</p></li>
  <li><p>what is the defaulValue argument of the QCommandLineOption
  constructor used for, anyway?</p></li>
  </ul>