Handle restructured text code-block syntax highlight

We are using restructuredtext to write our documentation and the code snippets always have a directive to hint how it should be rendered

.. code-block: python

    py = some.magic()

So far it does not seem that this directive is being processed at all.

Proposal

For ReStructuredText files:

1. Support .. code-block syntax highlighting. Code blocks should be rendered appropriately.
2. Code should be presented without the current background-color as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/20023#note_86125862 and in the sample readme in https://gitlab.com/gitlab-org/gitlab-ce/issues/20023#note_133302401.