Timedelta to_json with date_format='iso'
Is this formatting for timdelta columns intended / correct?

```python
In [12]: df = pd.DataFrame({"A": pd.date_range("2016", periods=4, freq='D'),
    ...:                    "B": pd.timedelta_range('1D', periods=4, freq='D')})

In [13]: df.to_json(date_format='iso')
Out[13]: '{"A":{"0":"2016-01-01T00:00:00.000Z","1":"2016-01-02T00:00:00.000Z","2":"2016-01-03T00:00:00.000Z","3":"2016-01-04T00:00:00.000Z"},"B":{"0":"1970-01-02T00:00:00.000Z","1":"1970-01-03T00:00:00.000Z","2":"1970-01-04T00:00:00.000Z","3":"1970-01-05T00:00:00.000Z"}}'
```

So a `pd.Timedelta('1D')` goes to `"1970-01-02T00:00:00.000Z"`.

The expected might be a [duration](https://en.wikipedia.org/wiki/ISO_8601#Durations) like

`"P1DT0H0M0S"`.

It seems to be reusing the logic for datetime columns and treating it as the equivalent int -> iso format. Ideally `to_json` will have a new paramater `timedelta_format` that takes the same values as `date_format`.