Add new builtin variables

[asg017]

Since this is running on atom, we could set up a background server that can provide data/API's to the observable runtime to enhance these notebooks.

For example, imagine a env builtin variable that looks up environmental variables, like:

token = env`API_TOKEN`

data = fetch(`https://some.server.com/data?token=${token}`)

Or maybe, a way to directly access files from the filesystem - like:

data = d3.csvParse(await fs(`/home/user/projects/weather/data.csv`))

Or maybe, re-write require to require js modules from the local filesystem (like a node_modules directory)

I'd love to hear suggestions for what are some other cool default variables that could exist!