Markdown plugin rewritten as cb_story

New version of the Python Markdown 2 plugin:

http://github.com/seanh/PyBlosxom-Python-Markdown-2-Plugin/blob/master/markdown-plugin.py

I've only tested this very briefly. The plugin contained quite a lot of
cruft, and I wanted to cut it down to the bare minimum. It was an entry
parser plugin, I rewrote it as a preformatter, then as a postformatter,
and finally as a cb_story which seems to be the best callback for this.
It means that the markdown plugin doesn't have to worry about parsing
the entry files as an entry parser would have to and can leave that to
pyblosxom (or some entry parser plugin), but the markdown plugin can
still be selective and only apply to entries with certain filename
extensions (preformatters and postformatters can't do this). The code is
much shorter now.

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