title: Joi validating time field
body: <p>There is object with a property time (22:30:00).</p>
  
  <pre><code>const schema = Joi.object.keys({
    ...
    transactionDate: Joi.date().required(),
    transactionTime: Joi.time().required(), // ???
    ...
  });
  </code></pre>
  
  <p>How to validate a time field using <code>Joi</code>?</p>