Display "copy to clipboard" button for code blocks

[marcelgerber]

It would be great if all code blocks put_code would come with a "Copy to clipboard" button in the corner, similar to how GitHub and other sites do it.
For example, on the demo page it would be great to have an easy way to copy the sample code. Either on by default or as an option.

I would be happy to contribute this feature, if you can point me in the right direction.