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

README.md

@@ -204,6 +204,7 @@ To test the theme, run `bundle exec rake preview` and open your browser at `http
 - [Gumshoe](https://github.com/cferdinandi/gumshoe)
 - [jQuery throttle / debounce](http://benalman.com/projects/jquery-throttle-debounce-plugin/)
 - [Lunr](http://lunrjs.com)
+- [Clipboard.js](https://clipboardjs.com)
 
 ## License
 
@@ -282,3 +283,7 @@ Pure Liquid Jekyll Table of Contents is distributed under the terms of the [MIT
 Minimal Mistakes incorporates [Lunr](http://lunrjs.com),
 Copyright (c) 2018 Oliver Nightingale.
 Lunr is distributed under the terms of the [MIT License](http://opensource.org/licenses/MIT).
+
+Minimal Mistakes incorporates [clipboard.js](https://clipboardjs.com/),
+Copyright (c) 2021 Zeno Rocha.
+Clipboard.js is distributed under the terms of the [MIT License](https://opensource.org/licenses/MIT).