fix(preboot): use CustomEvent rather than Event

* Use CustomEvent for PrebootComplete event notification to simplify
  polyfill usage and improve browser support.
* Check CustomEvent type before creating a CustomEvent using the
  constructor. If the type is not a function, a warning is logged
  in the console to notify the user that PrebootComplete event
  could not be dispatched.
* Addition of a section in the README.md documenting the need of a
  CustomEvent polyfill.

Closes #64

Author: Quovadisqc

README.md

@@ -219,3 +219,7 @@ res.render('index', (req, res) => {
   res.send(html);
 });
 ```
+
+#### Browser support
+
+If you wish to support Internet Explorer 9-11, you will need to include a [Polyfill](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/CustomEvent#Polyfill) for `CustomEvent`.

src/lib/api/event.replayer.ts

@@ -189,9 +189,15 @@ export class EventReplayer {
     prebootData.apps = [];
     this.clientNodeCache = {};
 
-    // sent event to documernt that signals preboot complete
-    const completeEvent = new Event('PrebootComplete');
-    doc.dispatchEvent(completeEvent);
+    // send event to document that signals preboot complete
+    // constructor is not supported by older browsers ( i.e. IE9-11 )
+    // in these browsers, the type of CustomEvent will be "object"
+    if (typeof CustomEvent === 'function') {
+      const completeEvent = new CustomEvent('PrebootComplete');
+      doc.dispatchEvent(completeEvent);
+    } else {
+      console.warn('Could not dispatch PrebootComplete event. You can fix this by including a polyfill for CustomEvent.');
+    }
   }
 
   setFocus(activeNode: NodeContext) {