Add support for CSS classes in code blocks

[javiereguiluz]

In a Symfony book we have this RST code:

The workflow starts with the creation of a Git branch:

.. code-block:: terminal
    :class: hide

    $ git branch -D sessions-in-db || true

.. code-block:: terminal

    $ git checkout -b sessions-in-db

This command creates a ``sessions-in-db`` branch from the ``master`` branch. It "forks" the code and the infrastructure configuration.

The :class: hide attribute is important because that command should not be displayed to the book reader. Thus, we add the hide CSS class and hide that item on the webpage.

However, this doesn't work because the RST parser seems to not propagate CSS classes in code blocks. This PR tries to fix that, but it's not working. It contains a failing test in terminal.rst/ terminal.html files. Thanks!

[javiereguiluz]

Ryan, thanks a lot for fixing this error. You really know the parser well 🙇 Let's merge this!