title: Add custom metadata or config to package.json, is it valid?
body: <p>I have seen (don't remember where) a package.json file with custom keys starting with an underscore:</p>
  
  <pre><code>{
      "name": "application-name"
    , "version": "0.0.1"
    , "private": true
    , "dependencies": {
        "express": "2.4.7"
      , "jade": "&gt;= 0.0.1"
    }
    , "_random": true
  }
  </code></pre>
  
  <p>Are you allowed to do this?  Is it still valid?  If this is allowed, is there any documentation on the rules?</p>
  
  <p>Thanks!</p>