Board & Pieces 0.6

• Add the ability to use different marks.

  • board's marked-squares argument can now be a dictionary.
  • Remove board's marking-color, marked-white-square-background, and marked-black-square-background arguments in favor of white-mark and black-mark.
  • Add a marks submodule containing some marks: fill, circle, and cross.

• Improve the look of arrows and replace board's arrow-stroke argument with arrow-fill and arrow-thickness.