Add "copy codeblock" for non-prism highlighting

[qjcg]

With params.docs.prism = true set in hugo.toml, there's a "copy codeblock" button on code blocks that's very convenient.

It would be awesome if the same "copy codeblock" feature was available with params.docs.prism = false as well!

[colinwilson]

This is definitely a nice feature to have for those who prefer Chroma over PrismJS for syntax highlighting. Will implement this soon.

Thanks for the feature request.