enhancement of color features

[SimonForest]

This PR enables a more refined handling of colors in diagrams. For example, one can do

\twocell{
  (label["T'","T'","\bar U"])
  *
  (1 * 2[color=red] * (0 -> 2)[cap,color=red])
  *
  (1 * 2[color=red] * label["R",color=red] * label["L",color=red])
  *
  (1 * 1[color=red] * (2 -> 1)[rectangle,"=",color=red] * 1[color=red])
  *
  (1 * 1[color=red] * label["U",color=red] * 1[color=red])
  *
  (1 * (2 -> 2)[rectangle,"\alpha",color=red] * 1[color=red])
  }

[smimram]

Thanks a lot!