Initialize audio buffer with button

This is needed as Chrome now blocks audio playback when no gesture has
been made by the user before playing the audio.
So we now have a "Start game" button to fix this.

src/guessnum.es6

@@ -48,32 +48,45 @@ function numberToPhonemes(number) {
  */
 function GuessNum(e) {
     const output = e.ownerDocument.createElement('pre');
+    const button = e.ownerDocument.createElement('button');
     const input  = e.ownerDocument.createElement('input');
+    const show = (e) => e.style.display = 'inline-block';
+    const hide = (e) => e.style.display = 'none';
+    let number
     e.appendChild(output);
+    e.appendChild(button);
     e.appendChild(input);
+    hide(input);
+    button.type='button';
+    button.innerText = 'Start game';
+    button.addEventListener('click', function() {
+      output.textContent = '';
+      number = Math.floor((Math.random() * 99) + 1);
+      say(GUESS_A_NUMBER_BETWEEN_0_AND_ONE_HUNDRED);
+      hide(button);
+      show(input);
+    });
     function say(phonemes, raw) {
       let text = phonemes;
       while (text.length < 256) {
         text += ' '
       }
-      Renderer(phonemes, {phonetic: true});
       if (raw) {
         output.innerText += "\n" + raw;
       }
+      Renderer(phonemes, {phonetic: true});
     }
     input.onkeydown = (e) => {
       if (e.keyCode === 13) {
         e.preventDefault();
         if (guess(parseInt(input.value))) {
-          number = Math.floor((Math.random() * 99) + 1);
           output.innerText = "\n" + output.innerText.split("\n").pop();
+          hide(input);
+          show(button);
         }
         input.value = '';
       }
     };
-    output.textContent = '';
-    let number = Math.floor((Math.random() * 99) + 1);
-    say(GUESS_A_NUMBER_BETWEEN_0_AND_ONE_HUNDRED);
 
   /**
    * Guess the number.