title: package.json for different environments?
body: <p>I have a node.js app which I've just deployed to Heroku which depends upon the <a href="https://github.com/LearnBoost/node-canvas" rel="noreferrer">canvas</a> module. However, that module requires the cairo graphics library and long story short, I need to use a precomipiled version which is included with <a href="https://github.com/elspoono/node-canvas-heroku" rel="noreferrer">this fork of canvas</a>.</p>
  
  <p>Normally, my package.json dependencies look like:</p>
  
  <pre><code>  "dependencies" : {
      "canvas"   :  "0.10.0",
      "express" :  "2.5.6",
      "jade" : "0.20.1"
    }
  </code></pre>
  
  <p>But in order to get my app to work on Heroku, I must pull in the fork from github</p>
  
  <pre><code>  "dependencies" : {
      "canvas"   :  "git://github.com/elspoono/node-canvas-heroku.git#master",
      "express" :  "2.5.6",
      "jade" : "0.20.1"
    }
  </code></pre>
  
  <p>Is it possible to have a "Heroku-only" version of package.json? Or is there another section in package.json where I can "override" the canvas dependency in dev environments?</p>