Allow reStructuredText configuration from _config.py

When writing posts with code snippets in rst format (e.g.:
**Java**

.. include::  ./snippets/HelloWorld.java
   :code: java
)

By default, docutils generates long css class names [1], which are incompatible with the generated pygments css files [2], which use the short version.

This pull request demonstrates how to fix the concrete problem I'm having, but I think a generic approach to configure these settings from _config (`blog.rst.syntax_highlight="short"`) would be a better solution.

So it's a pull request/pull request draft/feature suggestion - decide how do you want to handle it!


[1] http://docutils.sourceforge.net/docs/user/config.html#syntax-highlight
[2] http://pygments.org/docs/cmdline/#generating-styles