Exercise: improve the color selector

Take the examples/perlarrays/process_command_line.pl script and improve it in several ways:

  1. Check if the value given on the command line is indeed one of the possible values and don't let other colors pass.
  2. Allow a --force flag that will disregard the previously implemented restriction. Meaning
  3. --color Purple should still report error but
  4. --color Purple --force should accept this color as well.