title: Set ignore list for no-undef (Defining expected variables from other sources)
body: <p>I've got some js code that, during tests gets loaded in multiple parts, and gets concatenated and uglified for prod.</p>
  
  <p>I have a config file that defines a variable <code>myConfig</code> and have the business-logic script, that expects <code>myConfig</code> to be set and globally available.</p>
  
  <p>This is working fine, and there are no errors during dev or prod.</p>
  
  <p>The problem is that I'd like to use a <code>no-undef</code> eslint rule to catch other variables and function from missing declarations. So I'm looking for ways to define a set of expected variables. </p>
  
  <p>Is there a way to define such variables?</p>