title: how to implement IApplicationBuilder?
body: <p>I went through the documentation given in : 
  <a href="https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware" rel="nofollow noreferrer">https://docs.microsoft.com/en-us/aspnet/core/fundamentals/middleware</a></p>
  
  <p>I understood that IApplicationBuilder gives us the ability to plumb in different features in the pipeline.</p>
  
  <p>Is it possible to see the implementation of IApplicationBuilder in the framework ?</p>
  
  <p>If yes, can I write my own version of IApplicationBuilder and override the pipeline ?</p>