title: Python filter nested dict given list of key names
body: <p>Is there a way to filter a nested dict in Python, so I can see only the keys I'd specified ?
  Example:</p>
  
  <pre><code>x = {
    "field": [
      {
        "nm_field": "ch_origem_sistema_chave",
        "inf_tabelado": {
          "dropdown_value": "",
          "dropdown_key": "",
          "url_lista": "",
          "chave_relacional": ""
        },
      },
      {
        "nm_field": "ax_andamento_data",
        "inf_tabelado": {
          "dropdown_value": "",
          "dropdown_key": "",
          "url_lista": "",
          "chave_relacional": ""
        },
      }
    ],
    "_metadata": {
      "dt_reg": "22/01/2014 16:17:16",
      "dt_last_up": "10/04/2014 16:30:44",
    },
    "url_detalhes": "/DetalhesDocsPro.aspx",
    "url_app": "/docspro",
  }
  
  y = filter(x, ['dropdown_value', 'nm_field', 'url_app', 'dt_reg'])
  </code></pre>
  
  <p>Then var y would be something like: </p>
  
  <pre><code>{
    "field": [
      {
        "nm_field": "ch_origem_sistema_chave",
        "inf_tabelado": {
          "dropdown_value": "",
        },
      },
      {
        "nm_field": "ax_andamento_data",
        "inf_tabelado": {
          "dropdown_value": "",
        },
      }
    ],
    "_metadata": {
      "dt_reg": "22/01/2014 16:17:16",
    },
    "url_app": "/docspro",
  }
  </code></pre>
  
  <p>I've tried to do something using defaultdict, but had no success with lists at any level of recursion. Also I found dificulty while working with different data structures.</p>