title: Typescript augmentation
body: <p>I've been trying unsuccessfully to augment a type in agGrid with a few new properties. </p>
  
  <pre><code>import * as agGrid from 'ag-grid/main';
  
  declare module "ag-grid/main" {
     export interface ColDef {
         format: string;
         unitType: string;
     }
  }
  </code></pre>
  
  <p>Everything I have tried results in the original ColDef being overwritten and a build error of: Export declaration conflicts with exported declaration of 'ColDef'</p>