Support select language colorizing inside .. code:: and .. code-block:: directives

[tristanlatr]

From #491 (comment)

Right now, the code block will always the be colorized with python syntax, no matter what's the first argument passed to .. code::.

First of all, I'de suggest to simply disable the colorizing when the directive argument is set to unrecognized value.

Then, we can think about whether we want to use what's already integrated in docutils: https://pygments.org/ or use something else present in twisted maybe?