title: AddCommandLine not found .netcore 2.1
body: <p>While i'm trying to invoke AddCommandLine() function on ConfigurationBuilder am getting syntax error that specifying </p>
  
  <ul>
  <li>IConfigurationBuilder doesn't a definition or doesnot contain an extension method AddCommandLine accepting the first argument as IConfigurationBuilder </li>
  </ul>
  
  <p>Project was of .NetStandard 2.0</p>
  
  <p>Environment: </p>
  
  <ul>
  <li>.net core 2.1</li>
  <li>.net Standard 2.0</li>
  <li>OS : window 10</li>
  </ul>
  
  <p>code snippet : </p>
  
  <pre><code>var config = new ConfigurationBuilder()
              .AddEnvironmentVariables()
              .AddCommandLine(args)
              .Build();
  </code></pre>
  
  <p><strong>Note: Tried to use ConfigurationBuilder from Microsoft.Extensions.Configuration</strong></p>