Add a copy button to code cells in markdown

[BrooklinJazz]

What do we think of adding a copy button to code blocks in markdown cells?

Something like how GitHub handles it. I.e.

copy this text

I'm happy to contribute this feature if we are interested. Alternatively, I'm experimenting with making a Kino.JS or SmartCell and could make it an external library.

[josevalim]

Sounds good to me, a PR is welcome! Note we already do this for outputs, so it can be a reference. :)

[BrooklinJazz]

Awesome! It's late for me tonight but I will have a crack at this as soon as I have some free time tomorrow

[BrooklinJazz]

Apologies. I tried to find some free time today, but I have a lot on my plate with prep for Elixir conf. I may have to let this sit for a little while - if someone else would like to pick it up, feel free to, but I will definitely circle back to it as soon as possible.