title: How to use render-header function in table-column in Element library?
body: <p>I need to have a custom table header (title + svg + tooltip) for Element table.
  I am trying to use the 'render-header' function without luck.
  To be more specific - how to print the label + SVG (with tooltip on hover) for each column?</p>
  
  <p>Html:</p>
  
  <pre><code> &lt;el-table-column property="name" label="Indicator" :render-header="appendTip"&gt;
          &lt;/el-table-column&gt;
  </code></pre>
  
  <p>Script:</p>
  
  <pre><code>appendTip(h, { column }) {
        return h(
          "el-tooltip",
          {
            attrs: {
              effect: "dark",
              content: "Test",
              placement: "top"
            }
          },
          [h("el-button", ["Tooltip"])]
        );
  </code></pre>
  
  <p>Thanks.</p>