Added extension for PlantUML diagrams

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.