title: Validate only one single field of a bigger Joi schema
body: <p>I want to use <code>Joi</code> for form field validation and want to stick with one big schema object for validation of the whole form, yet I want to run single entry validation after one form field has been changed - i.e. after the first form field has recieved a value, I do not want to validate the complete form, but only the one field updated. I am envisioning something like</p>
  <pre><code>const schema = Joi.object({
      username: Joi.string()
          .alphanum()
          .min(3)
          .max(30)
          .required(),
  
      password: Joi.string()
          .pattern(new RegExp('^[a-zA-Z0-9]{3,30}$'))
          .required(),
  });
  const validationResult = schema.username.validate('Tommy');
  </code></pre>
  <p>Is that possible?</p>