Add formatting presets

[KCarlile]

Background

Formatting/style presets could be a handy feature for site builders to choose predefined themes, like black and white.

Acceptance Criteria

• Formatting styles are defined
• Site builders can easily apply a style to a diagram
  • ...or to all diagrams, if Allow for global (page-level) configuration options #7 is implemented

Approach

• Use an enum for style names
• Pass the enum to the method that will apply the styles
• Should this be an instance method (gdj1.setTheme(GDJTheme.Dark);) or a static method (GDJ.SetTheme(gdj1, GDJTheme.Dark);)?

Notes

TBD