How to set the system variable?

I'd like to change the system variables in `index.json`? E.g. `PAGER` or `EDITOR`.

Control over system variables can solve: #5 and #7.

I can't use `code` or `courseData` for intro, as they're run in foreground/background after the shell profile is loaded. Ideally I should expect some boot script, before the `.bashrc` is loaded.