title: Merging multiple TypeSafe Config files and resolving only after they are all merged
body: <p>I am writing test code to validate a RESTful service.  I want to be able to point it at any of our different environments by simply changing an environment variable before executing the tests.</p>
  
  <p>I want to be able to merge three different config files:</p>
  
  <ul>
  <li><code>conf/env/default.conf</code> - the default configuration values
  for all environments</li>
  <li><code>conf/env/&lt;env&gt;.conf</code> - the environment-specific
  values </li>
  <li><code>application.conf</code> - the user's overrides of any of the
  above</li>
  </ul>
  
  <p>The idea is that I don't want everything in a single config file, and run the risk of a bad edit causing configuration items to get lost.  So instead, keep them separate and give the user the ability to override them.</p>
  
  <p>Here's where it gets tricky:  <code>default.conf</code> will include ${references} to things that are meant to be overridden in <code>&lt;env&gt;.conf</code>, and may be further overridden in <code>application.conf</code>.  </p>
  
  <p>I need to postpone resolving until all three are merged.  How do I do that?</p>