support nested extensions

[ThomasPr]

The new extension feature since version 2.0.2 allows to define entries like this:

<info>
  <title>My API</title>
  <extensions>
    <x-my-extension>anything</x-my-extension>
  </extensions>
</info>

Unfortunately, nested values inside an extension are ignored and rendered as null values.

<info>
  <title>My API</title>
  <extensions>
    <x-my-extension>
      <nested>anything</nested>
    </x-my-extension>
  </extensions>
</info>

I kindly ask to add support for such nested extensions.

[lenny1806]

I would also really appreciate this functionality!