Added new plantUML contrib, changed graphviz and gnuplot contrib.

[woudshoo]

gnuplot contrib is changed:

• New variable allows to configure the gnuplot executable
• Allow terminal to be specified
• Default terminal depends on system
• Default is settable with configuration variable

graphviz contrib is implemented plantuml contrib is new

[eudoxia0]

Looks wonderful, thank you. And sorry this took so long to review! My GitHub notifications are a bit of a mess.