title: Joi validator only one of keys
body: <p>I am working on an api that should allow multiple params but for three of them I would like to allow only one of them.
  It easier with values for each key but I am wondering if Joi allows it too or I should add extra validation logic in my server. </p>
  
  <p>In short, for keys <code>a</code>, <code>b</code> or <code>c</code> I want to fail any request that has more than one of the three, so:</p>
  
  <ol>
  <li><p><code>http://myapi.com/?a=value</code> is <strong>a valid</strong> request.</p></li>
  <li><p><code>http://myapi.com/?b=value&amp;c=value2</code> is <strong>invalid</strong>.</p></li>
  </ol>
  
  <p>Thanks!</p>