title: Bootstrap-Vue: Table: Get index which doesn't change after sorting a column
body: <p>How to get an index like 0, 1, 2, 3... which doesn't change after sorting a column ?</p>
  
  <p>I have add the <code>primary-key</code> prop to <code>&lt;b-table&gt;</code>, but the initial index is reinitialized after a column is sorted (by clicking the arrays in a column title). </p>
  
  <pre><code>&lt;template&gt;
    &lt;div&gt;
      &lt;b-table striped hover :fields="fields" :items="items" primary-key&gt;
        &lt;!-- A virtual column --&gt;
        &lt;template slot="index" slot-scope="data"&gt;
          {{ data.index }}
        &lt;/template&gt;
      &lt;/b-table&gt;
    &lt;/div&gt;
  &lt;/template&gt;
  
  &lt;script&gt;
    export default {
      data() {
        return {
          fields: [
            "index",
            {
              key: 'last_name',
              sortable: true
            },
            {
              key: 'first_name',
              sortable: false
            },
            {
              key: 'age',
              label: 'Person age',
              sortable: true
            }
          ],
          items: [
            { age: 40, first_name: 'Dickerson', last_name: 'Macdonald' },
            { age: 21, first_name: 'Larsen', last_name: 'Shaw' },
            { age: 89, first_name: 'Geneva', last_name: 'Wilson' },
            { age: 38, first_name: 'Jami', last_name: 'Carney' }
          ]
        }
      }
   }
  &lt;/script&gt;
  </code></pre>