title: Nodejs - Joi Check if string is in a given list
body: <p>I'm using <a href="https://github.com/hapijs/joi/blob/v10.1.0/API.md" rel="noreferrer">Joi package</a> for server side Validation.<br> I want to check if a given string is in a given list or if it is not in a given list.(define black list or white list for values)<br>
  sth like an <em>"in"</em> or <em>"notIn"</em> function.how can I do that?</p>
  
  <pre><code>var schema = Joi.object().keys({
      firstname: Joi.string().in(['a','b']),
      lastname : Joi.string().notIn(['c','d']),
  });
  </code></pre>