Python-Markdown Parser, Extensions and Tables

Adding the original python-markdown parser ( 2.2.1 )
as an alternative to markdown2

See: https://github.com/waylan/Python-Markdown/

*python-markdown* is less monolithic than *markdown2*,
and is designed for extensibility.

Support for the table syntax that is used
in github flavoured markdown is enabled by default.

This commit introduces the following features:

- parser can be selected via the package settings
- set of extensions / extras can be configured
- lots of usage examples and links in the sample file