Add support for fenced code block

[michalbundyra]

Currently pyton markdown generator does not support fenced code blocks:
https://python-markdown.github.io/extensions/fenced_code_blocks/#syntax

Unfortunately in some docs we are using it and it causes a problem, as the code block is not readable.

We can add custom script which will fix it (temporarly - before version 3 of generator is landed - see: Python-Markdown/markdown#53)

Ref: zendframework/zend-view#196