consider supporting pandas DataFrame.to_json(orient='zipped') ?
I'm working with [vega-lite](https://github.com/uwdata/vega-lite) in a pipeline that goes from pandas to vega-lite. I would like to take a pandas DataFrame, output it as json, and incorporate it into a vega-lite JSON structure. Unfortunately none of pandas's DataFrame.to_json() "orient" options matches what vega-lite expects, so in the interest of better interoperability, I would like to request a 6th "orient" option, which I am calling 'zipped' but you could rename to anything.

Here is JSON for a vega-lite bar chart: https://github.com/uwdata/vega-lite#simple-bar-chart The "data" field is:

```
  "data": {
    "values": [
      {"x":"A", "y":28}, {"x":"B", "y":55}, {"x":"C", "y":43},
      {"x":"D", "y":91}, {"x":"E", "y":81}, {"x":"F", "y":53},
      {"x":"G", "y":19}, {"x":"H", "y":87}, {"x":"I", "y":52}
    ]
  },
```

I believe this is sort-of a dict(zip()) over a pandas DataFrame: every row in the DataFrame becomes one dict in the output list-of-dicts, with the keys in the dict equivalent to the column headings and the values in the dict equivalent to the values in the row associated with their respective column.

I am certainly aware that I could write code to manipulate an existing to_json() output, but it would seem this would be a useful output format for all.
