Make pandoc spit out "code-block" instead of "code"

[mgeier]

The code directive is not officially supported by Sphinx, therefore the highlight_language option I used in 77dc80e is not working.

Either this issue has to be fixed in Sphinx: sphinx-doc/sphinx#2155 (which, according to the comments, seems unlikely), or we have to make pandoc (which is used in markdown2rst) generate code-block instead of code directives.

All this only affects code within Markdown cells, the actual code cells are handled separately (not by pandoc).

For reference:
http://docutils.sourceforge.net/docs/ref/rst/directives.html#code
http://www.sphinx-doc.org/en/stable/markup/code.html