title: Joi validation multiple conditions
body: <p>I have the following schema:</p>
  
  <pre><code>var testSchema = Joi.object().keys({
      a: Joi.string(), 
      b: Joi.string(), 
      c: Joi.string().when('a', {'is': 'avalue', then: Joi.string().required()})
  });
  </code></pre>
  
  <p>but I would like to add a condition on <code>c</code> field definition so that it is required when: </p>
  
  <p><code>a == 'avalue' AND b=='bvalue'</code></p>
  
  <p>How can I do that?</p>