title: Allow optional parameters in Joi without specifying them
body: <p>I'm fairly new to using Joi to validate request payloads in hapi. My question is the following. I have this defined route:</p>
  
  <pre><code>{
      method: 'POST',
      path: '/foo/bar',
      config: {
        description: 'foo.bar',
        handler: handlers.foo,
        auth:false,
        tags: ['api'],
        validate: {
          payload: {
            email : Joi.string().required(),
            password : Joi.string().required(),
          }
        }
      }
  }
  </code></pre>
  
  <p>Email and password are my required properties. However, i would like to allow other properties without having to specify them all. for example:</p>
  
  <pre><code>{
    email: foo@bar.com,
    password: fooPass,
    name: myName,
    surname: mySurname
  }
  </code></pre>
  
  <p>Is there a way to do it with Joi?</p>