title: Joi - validate object keys and values?
body: <p>how could I use Joi to validate a substitutions field has zero or more key /value pairs ? and that each key is a string and that each value is a string, number or bool ?</p>
  
  <pre><code>"substitutions": {
      "somekey": "someval",
      "somekey": "someval"
    }
  </code></pre>