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

docs/_docs/05-configuration.md

@@ -296,6 +296,30 @@ For example,
 }
 ```
 
+### Code block copy button
+
+To enable a copy button on code blocks, add the following to `_config.yml`:
+
+```yaml
+enable_copy_code_button: true
+```
+
+When enabled site-wide, the button can be disabled on individual code blocks by adding `no-copy` to the code block's class list.
+
+````markdown
+```
+Hey, I have a "copy to clipboard" button!
+```
+````
+
+````markdown
+```
+But I don't have one.
+```
+{: .no-copy}
+````
+{: .no-copy}
+
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