Allow setting and overriding default environment variables per notebook.

[seivan]

I am not even sure if it's possible, but I don't mind taking a look at implementing it if needed.
That being said, it's only a problem because there are libraries that rely on env instead of letting us pass the value directly as a param.

Could be a menu button that opens up here

[josevalim]

This is better done with code: call System.put_env at the top of your notebook and be happy :)

[seivan]

I blanked! Sorry!