Preload bell audio

Allow the bellSound to be more easily configurable. You can pass false
to the bellSound and no sound will be played or you can pass it any
source you'd like.

src/InputHandler.ts

@@ -108,8 +108,8 @@ export class InputHandler implements IInputHandler {
    */
   public bell(): void {
     this._terminal.emit('bell');
-    if (this._terminal.playDefaultBell) {
-      (new Audio(Bell)).play();
+    if (this._terminal.bellSound) {
+      this._terminal.bellAudioElement.play();
     }
     if (!this._terminal.visualBell) {
       return;