title: using interpolation in .env files
body: <p>I'm trying to use a <code>.env</code> file in a node app and <code>dotenv</code> NPM module to read it, but use some variables and interpolation.
  what works in a standard bash file doesn't seem to run within a .env config file though. e.g., given:</p>
  
  <pre><code>APP_NAME=tixy
  MONGODB_URI="mongodb://127.0.0.1:27017/${APP_NAME}"
  </code></pre>
  
  <p>will come out directly in code</p>
  
  <pre><code>const mongoUri = process.env.MONGODB_URI
  </code></pre>
  
  <p>as <code>"mongodb://127.0.0.1:27017/${APP_NAME}"</code></p>
  
  <p>is there a way to get a <code>.env</code> config to run?
  perhaps I could 'source' it as the app starts up and use <code>export</code> for all the vars, but that seems kludgey...</p>