Generate a data loader file from a cell

[cinxmo]

Idea: Be able to define a data loader cell. For example:

```.py {name: requests.csv}
import requests
resp = results.get("https://random.com")
```

This will automatically generate a requests.csv.py file with the code and attempt to run it. It would be really cool if updating the code in requests.csv.py will reflect in this code block, but that sounds complicated...

[mbostock]

Closely related idea: “page fragment loader”.