Python markdown parser for Sublime Text 3.

This pull request add Python markdown (v2.3.1, see [Doc][1] and [Package][2])  parser which support for:

* Abbreviations
* Attribute Lists
* Definition Lists
* Fenced Code Blocks
* Footnotes
* Tables
* Smart Strong
* CodeHilite


This is a bit duplicated to #69. But that was for ST2.

Its ST3 now, the loading of python modules is simpler.

This patch has been tested on Windows.

[1]: http://pythonhosted.org/Markdown/
[2]: https://pypi.python.org/pypi/Markdown

TODO:

* [x] set of extensions / extras can be configured