fix: use more generic postcss-wrap

Author: e0ipso

postcss.config.js

@@ -1,3 +1,3 @@
 module.exports = {
-  plugins: [require('postcss-wrap')({ selector: '#example-calculator' })],
+  plugins: [require('postcss-wrap')({ selector: '.widget-instance' })],
 };

public/index.html

@@ -41,7 +41,7 @@ <h1 class="title">Widget Calculator</h1>
               </div>
               <div class="message-body">
                 <!-- START: WIDGET -->
-                <div id="example-calculator" data-button-text="Click here">
+                <div id="example-calculator" class="widget-instance" data-button-text="Click here">
                   <progress class="progress is-small is-primary" max="100">0%</progress>
                 </div>
                 <!-- END: WIDGET -->