title: How to add Mobx observer to LitElement
body: <p>I have the following component, my component correctly displays the message from appState but when I change the value of appState the component isn't updated. I know I need to add an @observer, but how do you add it to a LitElement?</p>
  
  <pre><code>import { LitElement, html } from 'lit-element';
  import { observable } from "mobx";
  
  var appState = observable({
    message: 'World'
  });
  
  class MyElement extends LitElement {
  
    handleClick() {
      appState.message = 'All';
    }
  
    render(){
      return html`
        &lt;p&gt;Hello, ${appState.message}&lt;/p&gt;
        &lt;button @click=${this.handleClick}&gt;Click me&lt;/button&gt;
      `;
    }
  }
  
  customElements.define('my-element', MyElement);
  </code></pre>