Add copy button to copy code block contents (#345)

* add buttons to copy code block contents

Adds a clickable "copy" link in the top-right corner of each code block.

If available, uses the navigator.clipboard API. Falls back to selecting
the text and calling document.execCommand('copy') to copy text.

* hides copy button unless mouse is hovering over code block

* change text of copy button when text is copied

* add translation keys for copy button text  `code_copy` and `code_copied`

* To disable use `Params.disableCodeCopy: true` in site config

layouts/partials/footer.html

@@ -84,3 +84,43 @@
 
 </script>
 {{- end }}
+
+{{- if (not .Site.Params.disableCodeCopy) }}
+<script>
+    document.querySelectorAll('pre > code').forEach((codeblock) => {
+        const container = codeblock.parentNode.parentNode;
+
+        const copybutton = document.createElement('button');
+        copybutton.classList.add('copy-code');
+        copybutton.innerText = '{{- i18n "code_copy" | default "copy" }}';
+
+        function copyingDone() {
+            copybutton.innerText = '{{- i18n "code_copied" | default "copied!" }}';
+            setTimeout(() => {
+                copybutton.innerText = '{{- i18n "code_copy" | default "copy" }}';
+            }, 2000);
+        }
+
+        copybutton.addEventListener('click', (cb) => {
+            if ('clipboard' in navigator) {
+                navigator.clipboard.writeText(codeblock.textContent);
+                copyingDone();
+                return;
+            }
+
+            const range = document.createRange();
+            range.selectNodeContents(codeblock);
+            const selection = window.getSelection();
+            selection.removeAllRanges();
+            selection.addRange(range);
+            try {
+                document.execCommand('copy');
+                copyingDone();
+            } catch (e) { };
+            selection.removeRange(range);
+        });
+
+        container.appendChild(copybutton);
+    });
+</script>
+{{- end }}