title: Read line from file and load it to actual config
body: <p>I'm using the <a href="https://typesafehub.github.io/config/latest/api/com/typesafe/config/ConfigFactory.html" rel="nofollow noreferrer">ConfigFactory</a> class to load the config of my application, but i want to know how can I load a line from other file and add it to the Config object.
  The file only contains 1 line with the value I want, and i want to add that value with to the config with a new key or better to a existing one, so i can have a default value.</p>
  <p>Is it possible to add a custom key/value after the config is loaded?, like:</p>
  <p><code>config.add(&quot;key&quot;, &quot;value&quot;)</code></p>