bin/colortable: Print a color table of ANSI colors (normal+bright)

* Introduce bin/colortable which prints a grid of all the standard
  ANSI colors for each possible foreground + background combo,
  including handling for no-background and bright fore+back colors.
* Delete bin/colordump because it wasn't a useful 256 color utility
  and I'm not using any 256 color terminals currently.