added an example page for Markdown syntax

Sadly Markdown does not support all that Textile supports. In order to
better understand the possibilities I added a reference article content
that should make it easier for people coming from Textile to have a
reference.
Markdown Extra 0.1

First shot at adding Markdown Extra - probably cannot be active at the
same time as wet_textfilter_markdown - perhaps it would be nice to have
wet_textfilter_markdown_extra also parse wet_markdown type texts...
updated example article
Merge branch 'master' of https://github.com/perlmonkey/wet_textfilter_markdown into markdownextra
Merged Markdown Extra. Tagged v0.4.