title: How to update (or add/remove) all annotations when using angular-highcharts
body: <p>I want to add annotations to a my line chart after the chart has been built. I used <a href="https://www.highcharts.com/demo/chart-update" rel="nofollow noreferrer">this</a> example for updating options. This works fine for other options, but not annotations. The following will update <code>subtitle</code>, but not <code>annotations</code>.</p>
  
  <pre><code>console.log(this.chart); // Shows no annotation labels, as expected.
  const labels = this.createAnnotationLabels();
  console.log(labels); // Show actual labels
  const optionsUpdate: any = {
    subtitle: {
      text: new Date()
    },
    annotations: [{
      labels: labels,
      labelOptions: {
        align: 'right',
        allowOverlap: true
      }
    }]
  };
  this.chart.ref.update(optionsUpdate);
  console.log(this.chart); // Unexpectedly shows no annotation labels anywhere.
  </code></pre>
  
  <p>If I add the annotations as an option before I create the chart however, then the annotations show up fine.</p>
  
  <p>How should I go about removing/updating/adding annotations after a chart has been built? I've got one version where I reconstruct the whole thing, but that becomes a very jarring user experience.</p>
  
  <p>My <code>package.json</code> file includes:</p>
  
  <pre><code>{
      "@angular/common": "5.2.9",
      ...
      "angular-highcharts": "5.2.12",
      "highcharts": "6.1.0"
  }
  </code></pre>