如何在 nodejs 使用环境变量

https://github.com/ascoders/weekly/blob/master/%E5%89%8D%E6%B2%BF%E6%8A%80%E6%9C%AF/60.%E7%B2%BE%E8%AF%BB%E3%80%8A%E5%A6%82%E4%BD%95%E5%9C%A8%20nodejs%20%E4%BD%BF%E7%94%A8%E7%8E%AF%E5%A2%83%E5%8F%98%E9%87%8F%E3%80%8B.md