Add "copy to clipboard" button for code blocks (#2812)

* Add copy-to-clipboard button and JS

* Ignore line numbers if present

* Rewrite heading permalink code to use vanilla JS

* README: Add credits to zenorocha/clipboard.js (MIT License)

@iBug really wants a place here in the Credits section :P

* Add .no-copy for hiding the button, update docs

* Add td.rouge-code to selectors

* Fix navigator.clipboard branch

* Add screenreader text for copy button

* Restore focus to the button after copying

* Add site-wide enable switch

_includes/head.html

@@ -9,8 +9,11 @@
 {%- comment %} https://docs.google.com/presentation/d/1rmxwWa9P6_xHqonmh5ONXRS-jPc5XKbnv99Rjkhe04s/present {% endcomment -%}
 <meta name="viewport" content="width=device-width, initial-scale=1.0">
 
-<script>
+<script type="text/javascript">
   document.documentElement.className = document.documentElement.className.replace(/\bno-js\b/g, '') + ' js ';
+  {% if site.enable_copy_code_button -%}
+    window.enable_copy_code_button = true;
+  {%- endif %}
 </script>
 
 <!-- For all browsers -->