title: JSON Schema dependencies: enum content depends on choice from another enum
body: <p>I have an <code>enum</code> with a first set of choices, and a second <code>enum</code> whose content depends on the choice made in the first <code>enum</code>.</p>
  
  <p>Here's a simple example of what I have at present (not right):</p>
  
  <pre><code>"fontGroup": {
      "title": "Font Group",
      "type": "string",
      "enum": [
          "Roboto",
          "Noto",
          "Alegreya"
      ],
      "default": "Roboto"
  },
  "fontFamily": {
      "title": "Font Family",
      "type": "string",
      "enum": [
          "Roboto Slab",
          "Roboto Condensed",
          "---",
          "Noto Sans",
          "Noto Serif",
          "---",
          "Alegreya SC",
          "Alegreya Sans"
      ],
      "default": "Roboto Slab"
  }
  </code></pre>
  
  <p>Of course, if <code>Noto</code> is selected from the first <code>enum</code> then only the <code>Noto</code> related choices in the second <code>enum</code> are valid.  It is invalid to select <code>Noto</code> in combination with <code>Roboto Condensed</code>, for example.</p>
  
  <p>How can this be specified in the schema?</p>