Add copy to clipboard to code blocks

layouts/_default/_markup/render-codeblock.html

@@ -1,5 +1,13 @@
 <div class="code-block">
-    <div class="code-type">{{ .Type }}</div>
+    <div class="code-header">
+        <span class="code-type">{{ .Type }}</span>
+        <a class="code-copy-button">
+            {{ i18n "copy_to_clipboard" }}
+        </a>
+    </div>
     {{/* a div.highlight is already created by highlight function */}}
     {{ highlight .Inner .Type }}
+
+    {{/* for copy to clipboard */}}
+    <pre class="code-raw" style="display: none;">{{ .Inner }}</pre>
 </div>