title: Extending @types - delete field from interface, add type to field in interface
body: <p>I have javascript library with types from npm/@types. </p>
  
  <p>I need to make two fixes to @types which applies only in case of my application, so I can't merge them into DefinitelyTyped repository.</p>
  
  <p>I need to:</p>
  
  <ol>
  <li><p>remove one of fields from interface. Example:</p>
  
  <pre><code>// before changes:
  interface A {
          a?:string;
          b?:string;
          c?:string;
  }
  
  // after changes:
  interface A {
          a?:string;
          c?:string;
  }
  </code></pre></li>
  <li><p>add more types to one field in interface. Example:</p>
  
  <pre><code>// before changes:
  interface B {
          a?: C;
  }
  
  // after changes:
  interface B {
          a?: C | D;
  }
  </code></pre></li>
  </ol>
  
  <p>Also I still want to download main @types definitions from external repository.</p>
  
  <p>What is the best way to achieve this?</p>