title: Validation on relation between parameters, with hapijs/joi
body: <p>I am trying to validate optional parameters from a querystring that are depending on each other. In other words, when parameter A is added, parameter B is also needed. Both parameters are optional, so when neither of them are provided validation should also pass.</p>
  
  <p>Should pass:</p>
  
  <pre><code>?a=1&amp;b=2&amp;c=0
  ?c=0
  </code></pre>
  
  <p>Should fail:</p>
  
  <pre><code>?a=1&amp;c=0
  ?b=2&amp;c=0
  </code></pre>
  
  <p>Can someone give me an example schema on how to solve this?</p>