Enhancement - Color::colorize

- Made Color::colorize (const std::string&, const std::string&) static,
  so that it is now simple to colorize a string:

      std::cout << Color::colorize ("text", "red");

- Added sample foreground and background colors to the "color" command,
  and used the above shortcut to simplify handleOnColor.

src/Color.h

@@ -60,7 +60,7 @@ public:
   void blend (const Color&);
 
   std::string colorize (const std::string&);
-  std::string colorize (const std::string&, const std::string&);
+  static std::string colorize (const std::string&, const std::string&);
 
   bool nontrivial ();