Dynamic headers

Add ability to dynamically add headers from a MongoDB or PHP Report when using datasets.  Here's an example:

```js
printjson([
  {
    title: "Dataset 1",
    headers: [
      {
        "header": "Chart",
        "value": {

        },
        "rows": [
        
        ]
      }
    ]
  }
]);
```

For headers that are dataset aware (Formatting, Chart, Rollup, etc.), the `dataset` property is automatically set before parsing the header.