Global preamble#5

The default PlantUML style is colorful, but not good for printouts. Currently, the style has to be put at each `plantuml` environment. There should be a global configuration put just before the code in the `plantuml` environment. Example preamble: ``` 'skinparam monochrome true 'Chooses LightGray as background color 'We're better off with manual setting skinparam class { BackgroundColor White ArrowColor Black BorderColor Black LegendBackgroundColor White } skinparam stereotypeCBackgroundColor LightGray skinparam noteBackgroundColor White skinparam noteBorderColor Black 'required for SVG skinparam defaultFontName sans-serif ```