title: How to setup environment variables in Brunch
body: <p>We're developing an app using Chaplin on top of brunch.</p>
  
  <p>We're consuming a 3rd party API and we have <strong>different tokens for development and production</strong>.</p>
  
  <p>I was wondering if it's possible to setup env variables in Brunch. I'd like to compile everything to plain html/js. </p>
  
  <p>Is there a way to compile the project using something like <code>brunch build -env staging</code>? If so, how can I define such variables?</p>