Futher improve colors

• Add -color option to specify a color spec alongside SAC_COLORS.
  • The -color option takes precedence over the SAC_COLORS env var.
• Remove the global struct and global variables from color.h
  • The color count variables where moved to getter functions that return a constant based on sizeof.
  • The struct definition is moved to types.h and now has a typedef to color.
• Improve -help colors formatting and readability.
  • Bring formatting inline with other help pages
  • Calculate the default value of the color spec

The table of color struct is still in color.c, but now we access it through global.colors. This is done by assigning global.colors (which is initialized as NULL) to COLORS in COLORinit. The same is done for global.named_colors and NAMED_COLORS.