Cannot handle `default` which is an object

[abemedia]

I have a schema which has defaults that are objects e.g.

    default:
      foo: bar

The resulting docs do not show JSON for this, but simply the following:

    <tr>
      <th>Default</th>
      <td colspan="2">[object Object]</td>
    </tr>

[tomcollins]

This will need an update to lib/renderer/index.js to handle objects.