title: Joi object key type
body: <p>I have object with dynamic string keys with string values, </p>
  
  <pre><code>{
    [string]: string
  }
  </code></pre>
  
  <p>how can I do this?</p>
  
  <pre><code>Joi.object().keys({
    [Joi.string()]: Joi.string()
  })
  </code></pre>
  
  <p>not working :(</p>