"export data" helper

Some Jupyter users (and I've been in this camp) will write a notebook simply to transform data and export it so they can use the data in some other system. It might be nice to provide some simple API functionality to export your data as a csv or json efficiently, so a user doesn't necessarily need to figure out how to do that w/ lower level system calls. The calls could look like:

```javascript
iodide.file.exportAsCSV(data, 'out.csv')
iodide.file.exportAsJSON(data, 'out.json')
iodide.file.exportAsBinary(data, 'out.whatever')
// the functions above serve as convenience functions for this one:
iodide.file.export(data, Array.isArray(data) ? : 'csv' : 'json', 'out.data')
iodide.file.export(imgData, 'binary', 'out.imgData')
```

Or something like that.