title: Boost program options - get all entries in section
body: <p>According to documentation I can parse config files in style :</p>
  
  <pre><code> [main section]
   string = hello world. 
   [foo]
   message = Hi !
  </code></pre>
  
  <p>But I need to parse list of plugins :</p>
  
  <pre><code> [plugins]
   somePlugin. 
   HelloWorldPlugin
   AnotherPlugin
   [settings]
   type = hello world
  </code></pre>
  
  <p>How can I get vector of strings which are in plugins section ?</p>