Fix clipboardjs implemention

Previous commit 4fb0adb24a didn't work
Credit to https://www.jianshu.com/p/3e9d614c1e77

themes/typing/source/css/typing.css

@@ -983,3 +983,25 @@ pre .javascript .function,pre .keyword{
         font-size:0.9em
     }
 }
+.copy-button {
+    cursor: pointer;
+    border: 1px solid #d5d5d5;
+    border-radius: 3px;
+    -webkit-user-select: none;
+    -moz-user-select: none;
+    -ms-user-select: none;
+    user-select: none;
+    -webkit-appearance: none;
+    font-size: 0.8em;
+    font-weight: 700;
+    line-height: 1em;
+    font-family:Courier,Courier New,monospace;
+    -webkit-transition: opacity .3s ease-in-out;
+    -o-transition: opacity .3s ease-in-out;
+    transition: opacity .3s ease-in-out;
+    padding: 2px;
+    opacity: 0;
+}
+.highlight:hover .copy-button{
+  opacity: 1;
+}

themes/typing/source/js/typing.js

@@ -22,20 +22,20 @@
     $('.fancybox').fancybox()
   }
 
+  //Add "Copy" button to code snippet
+  var code = document.getElementsByClassName('code');
 
-  $(document).ready(function() {
+  for (var i = 0; i < code.length; i++) {
-
-    // Add "Copy" button to code snippet
-    var pre = document.getElementsByTagName('pre');
-
-    for (var i = 0; i < pre.length; i++) {
     var button = document.createElement('button');
       button.className = 'copy-button';
       button.textContent = 'Copy';
 
-        pre[i].appendChild(button);
+      code[i].appendChild(button);
   }
 
+  $(document).ready(function() {
+
+    // Add copy to clipboard button for code snippet
     var copyCode = new ClipboardJS('.copy-button', {
         target: function(trigger) {
             return trigger.previousElementSibling;
@@ -74,4 +74,3 @@
     });
   });
 })(jQuery)
-