title: how to have custom attribute in ConfigurationElementCollection?
body: <p>for configuration as following</p>
  
  <pre><code>&lt;MyCollection default="one"&gt;
    &lt;entry name="one" ... other attrubutes /&gt;
    ... other entries
  &lt;/MyCollection&gt;
  </code></pre>
  
  <p>when implement a MyCollection, what should i do for the "default" attribute?</p>