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

_config.yml

@@ -29,6 +29,7 @@ logo                     : # path of logo image to display in the masthead, e.g.
 masthead_title           : # overrides the website title displayed in the masthead, use " " for no title
 # breadcrumbs            : false # true, false (default)
 words_per_minute         : 200
+enable_copy_code_button  : # true, false (default)
 copyright                : # "copyright" name, defaults to site.title
 copyright_url            : # "copyright" URL, defaults to site.url
 comments: