md-data-table支持colspan

Question

      <table md-table>
        <thead ng-if="!options.decapitate" md-head md-order="query.order" md-on-reorder="logOrder">
          <tr md-row>
            <th md-column md-numeric md-order-by="fat.value"><span>Fat (g)</span></th>
            <th md-column md-numeric md-order-by="carbs.value"><span>Carbs (g)</span></th>
            <th md-column md-numeric md-order-by="protein.value"><span>Protein (g)</span></th>
            <th md-column md-numeric md-order-by="sodium.value" hide-gt-xs show-gt-md><span>Sodium (mg)</span></th>
            <th md-column md-numeric md-order-by="calcium.value" hide-gt-xs show-gt-lg><span>Calcium (%)</span></th>
          </tr>
        </thead>
        <tbody md-body>
          <tr md-row ng-repeat="dessert in desserts.data">
            <td md-cell>{{dessert.calories.value}}</td>
            <td md-cell>{{dessert.fat.value | number: 2}}</td>
            <td md-cell>{{dessert.carbs.value}}</td>
            <td md-cell>{{dessert.protein.value | number: 2}}</td>
            <td md-cell hide-gt-xs show-gt-md>{{dessert.sodium.value}}</td>
          </tr>
        </tbody>
      </table>

如何将任意两列合并为md数据表angularjs中的一列，就像 bootstrap的colspan-2 一样？