title: Json schema require patternProperty
body: <p>Is there way to require patternProperty?</p>
  
  <pre><code>{
    "type": "object", 
    "$schema": "http://json-schema.org/draft-04/schema", 
    "patternProperties": {
      "^proper_.+_ty$": {
        "type": "string"
      }
    }
  }
  </code></pre>
  
  <p>Can I make this "^proper_.+_ty$" to be required or require arbitrary key in any other way?</p>