title: typescript type object with string keys and content defined by interface
body: <p>my data has this structure:</p>
  
  <pre><code>groups = {
      someGroupName: {
          total: 30,
          count: 3,
          average: 10
      },
      someOtherGroupName: {
          total: 60,
          count: 3,
          average: 20
      }
  }
  </code></pre>
  
  <p>I can write an interface for the nested part:</p>
  
  <pre><code>interface Stats {
      total: number,
      count: number,
      average: number
  }
  </code></pre>
  
  <p>but how can I set a type for groups as a whole? I do not know what groups are coming in. I just know that it will be a groupname as the key and the stats object as the value.</p>