How to setup environment variables in Brunch

We're developing an app using Chaplin on top of brunch.

We're consuming a 3rd party API and we have different tokens for development and production.

I was wondering if it's possible to setup env variables in Brunch. I'd like to compile everything to plain html/js.

Is there a way to compile the project using something like brunch build -env staging? If so, how can I define such variables?