title: Can't read custom environment variables in node using config.get()
body: <p>I use <a href="https://www.npmjs.com/package/config" rel="nofollow noreferrer">config library</a> to manage my configuration files in node application.</p>
  
  <p>I have export my custom env variable in terminal by 
    <code>export app_password=12345</code></p>
  
  <p>and map it into the <code>custom-environment-variable.json</code> file by </p>
  
  <pre><code>{
    "mail": {
    "password": "app_password"
     }
  }
  </code></pre>
  
  <p>when I use <code>config.get('mail.password')</code> in my index.js file (root module) .  </p>
  
  <p>I got <code>error: Configuration property "mail.password" is not defined</code>, detail in below:</p>
  
  <pre><code>/Users/tzhong/Documents/vidly- 
  backend/node_modules/config/lib/config.js:203
  throw new Error('Configuration property "' + property + '" is not 
  defined');
  ^
  
  Error: Configuration property "mail.password" is not defined
      at Config.get (/Users/tzhong/Documents/vidly- 
   backend/node_modules/config/lib/config.js:203:11)
  </code></pre>
  
  <p>I have use <code>console.log(process.env.app_password)</code> to check if my custom env variable is there, and the app_password show in the terminal. </p>
  
  <pre><code> Vidly evn is : Vidly backend -- development
   Vidly mail server is : dev-mail-server
   Mail-password is : 12345
   NODE_ENV is : undefined
   app: development
   Mongan enabled...
   Listenning 3000 port ...
  </code></pre>
  
  <p>I want to use <code>console.log(config.get('mail.password'));</code> to show my custom variable in terminal instead of <code>console.log(process.env.app_password)</code>
  Any suggestions?</p>
  
  <p>Finally solve the problem :
  It's tiny mistake, in config library, the file's name for creating custom environment variable is strict, the file's name should be <code>'custom-environment-variables.json'</code>, I have a wrong spell on <code>'environment'</code> , after all , thanks for people offer other available package </p>