Add a clipboard target as source of text for the copy to clipboard button

I'd like a simple declarative way to point to another element as the source of the text for the copy to clipboard button.

That way I don't have to muck around with the `setText` method. This PR is a very simplistic approach to it meant to start a conversation about the idea.

For example, it assumes you want `innerHTML` and not the `innerText` or the `value` attribute of the target element. We could add more options or infer this info based on the target element type.