title: How can I use labels to filter issues on the github issue tracker?
body: <p>I'm using the github issue tracker for the first time, and I'm trying to manage a set of about 50 open issues.   I would like to filter the set using standard Boolean queries over labels.  But all I can figure out how to do is AND queries.
  For example, I can show all issues that are labelled both <code>view/controller</code> and <code>easy meat</code>.  But I do not know how to do any of the following queries:</p>
  
  <ul>
  <li><p>Show me all open issues that are labeled <code>view/controller</code> but are <em>not</em> labeled <code>easy meat</code>.</p></li>
  <li><p>Show me all open issues that are labeled either <code>major refactoring</code> or <code>needs thought</code>.</p></li>
  <li><p>Show me every open issue that does not have any label.</p></li>
  </ul>
  
  <p>I've searched and I've RTFM, and I can't find a way to ask these kinds of queries.  <strong>Are such queries even possible?</strong>  If so, how does one ask them?</p>