title: How do I get the v-data-table to render icon for - and + in vuetify?
body: <p>I'm working on my vue.js project and I'm using Vuetify.
  Now, I've made a datatable like in this example from the docs: </p>
  
  <pre><code>https://codepen.io/pen/?&amp;editable=true&amp;editors=101
  </code></pre>
  
  <p>I'd like to have the same table with the - and + buttons (as you see in the code pen, around the category name).</p>
  
  <p>When I inspect the code in my browser, there seems to be some kind of buttons, but they don't show the buttons in the codepen example template.</p>
  
  <p>Does anyone know how to implement that?</p>
  
  <p>here is the code from the codpen:</p>
  
  <pre><code>&lt;div id="app"&gt;
    &lt;v-app id="inspire"&gt;
      &lt;v-data-table
        :headers="headers"
        :items="desserts"
        item-key="name"
        group-by="category"
        class="elevation-1"
        show-group-by
      &gt;&lt;/v-data-table&gt;
    &lt;/v-app&gt;
  &lt;/div&gt;
  
  new Vue({
    el: '#app',
    vuetify: new Vuetify(),
    data () {
      return {
        headers: [
          {
            text: 'Dessert (100g serving)',
            align: 'left',
            value: 'name',
          },
          { text: 'Category', value: 'category' },
        ],
        desserts: [
          {
            name: 'Frozen Yogurt',
            category: 'Ice cream',
          },
          {
            name: 'Ice cream sandwich',
            category: 'Ice cream',
          },
          {
            name: 'Eclair',
            category: 'Cookie',
          },
          {
            name: 'Cupcake',
            category: 'Pastry',
          },
          {
            name: 'Gingerbread',
            category: 'Cookie',
          },
          {
            name: 'Jelly bean',
            category: 'Candy',
          },
          {
            name: 'Lollipop',
            category: 'Candy',
          },
          {
            name: 'Honeycomb',
            category: 'Toffee',
          },
          {
            name: 'Donut',
            category: 'Pastry',
          },
          {
            name: 'KitKat',
            category: 'Candy',
          },
        ],
      }
    },
  })
  </code></pre>
  
  <p>My code looks like the example, except for the - and + buttons that has a wierd text instead of the icons as the example shows.</p>