title: Stripping unknown keys when validating with Joi
body: <p>I'm using Joi to validate a JavaScript object in the server. The schema is like the following:</p>
  
  <pre><code>var schema = Joi.object().keys({
      displayName: Joi.string().required(),
      email: Joi.string().email(),
      enabled: Joi.boolean().default(false, "Default as disabled")
  }).unknown(false);
  </code></pre>
  
  <p>The schema above will report an error if there is an unknown key in the object, which is expected, but what I want is to strip all the unknown silently, without an error.  Is it possible to be done?</p>