title: Node generate-schema missing additionalProperties
body: <p>I am using <a href="https://www.npmjs.com/package/generate-schema" rel="nofollow noreferrer">generate-schema</a> to generate json schema for my data object. However, I am getting all useful properties except for <strong>additionalProperties</strong>. How to add this to the generate schema?</p>
  
  <p>I am using it as below. Here dataObj is an object</p>
  
  <pre><code>var GenerateSchema = require('generate-schema');
  
  var schema = GenerateSchema.json(dataObj); 
  </code></pre>