Added extension for PlantUML diagrams

[mikitex70]

Added plugin for using PlantUML diagrams with Markdown.
Syntax:

::uml:: [format="png|svg"] [classes="class1 class2 ..."] [alt="text for alt"]
PlantUML script diagram
::end-uml::

Example:

::uml:: format="png" classes="uml myDiagram" alt="My super diagram"
Goofy -> MickeyMouse: calls
Goofy <-- MickeyMouse: responds
::end-uml::

See the plugin for detailed usage and installation of required tools.

[coveralls]

Coverage decreased (-2.5%) when pulling 6c6794b on mikitex70:master into 99fe8e2 on waylan:master.

[mitya57]

Needs to be compatible with Python 3:

except Exception as exc:

[mitya57]

Why does this need to be in Python-Markdown? Can it be a third-party extension instead?

If you create a separate repository for it, we can add it to the list of known extensions in the wiki.

[waylan]

@mikitex70 thanks for contribution. However, I agree with @mitya57, this should be a third party extension. I would suggest creating your own repo, push this extension to it, and add it to the list. The wiki is publicly editable, so feel free to add it yourself.

[mikitex70]

Sorry, I'm new to the Python Markdown code (and Python also). I've developed the plugin for Pelican RST syntax and someone has requeste me also a Markdown version. So I've thinked this was the best place where to put it.
I will make a third party extension and register to the wiki.
Thanks for your support.