Add "code-output" class to code-output blocks

[tlienart]

Now:

julia> s = """
       ```!
       1+1
       ```
       """ |> fd2html |> println
<pre><code class="language-julia">1&#43;1</code></pre>
<pre><code class="plaintext code-output">2</code></pre>

so same as before but there's the extra class code-output that allows separate styling of code output blocks.

closes #856

[rikhuijzer]

Nice feature! Feel free to copy the CSS from Books.jl for output blocks 😄 From the docs:

EDIT: CSS at https://github.com/rikhuijzer/Books.jl/blob/d01e0480238507e15327270e6c3d3bf3a6038b2c/defaults/style.css#L273-280

[tlienart]

@rikhuijzer would you mind opening an issue on FT for this please?