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

i18n/en.yaml

@@ -17,3 +17,9 @@
 
 - id: home
   translation: "Home"
+
+- id: code_copy
+  translation: "copy"
+
+- id: code_copied
+  translation: "copied!"