Fix bug #2511: A lot of code examples in documentation not highlighted

This merge request adds monkey-patch to Sphinx that forces it to output error message if any of code blocks are unhighlighted.

For blocks that doesn't require highlighting should be used following block specification:

    .. code-block:: none

In Python code it is possible to use ellipsis `...` as expression placeholder, but beware that Sphinx handles it "intelligently", so sometimes its use can be tricky. For details see sphinx source code [1].

[1] https://bitbucket.org/birkenfeld/sphinx/src/e5ad8ce30a8a4795757add8e736c8114ca9ee38b/sphinx/highlighting.py?at=default#cl-128