Output JSON

Support for outputting json from datapipes.

Default output format is:

```
[
  { field: value, field: value },
  ...
]
```
### Other possible output formats (future work)

```
[
  [ value, value, value, ... ]
]
```

"Full"

```
[
  {
    index: 0,
    row: { .... }
  },
  ....
]
```
## Implementation

Probably want to use https://github.com/dominictarr/JSONStream so we stream ...
