graph - allow graphviz colorscheme / layout customization

[tintinweb]

!! not sure if I messed up the original coloring and my change is pretty hacky. maybe you have a better idea on how to let users provide the colorscheme.

• optionally allow user to customize graph colorscheme; default is still the original light colorscheme.
• added dark colorscheme
  • node color = internal/private (orang'ish), public/external (red'ish)
  • edge color = green (internal); white (all others)
  • modifiers have their own shape
  • unknown contracts have a dashed border
• added contract kind to cluster label
• allows to highlight payable nodes

see example using defaultColorSchemeDark

[GNSPS]

Woohoo! 🎉 LGTM