Missing Colorize::Mode values

[HertzDevil]

As suggested in https://forum.crystal-lang.org/t/italic-mode-not-supported-in-colorize/5890, several SGR codes from ECMA 48 are not defined in the Colorize::Mode enum yet, presumably because they weren't widely supported. The flags that don't simply cancel other flags' effects are:

• \e[3m: italicized
• \e[6m: rapidly blinking (150 per minute or more)
• \e[9m: crossed-out (characters still legible but marked as to be deleted)
• \e[20m: Fraktur (Gothic)
• \e[21m: doubly underlined
• \e[51m: framed
• \e[52m: encircled
• \e[53m: overlined
• \e[60m: ideogram underline or right side line
• \e[61m: ideogram double underline or double line on the right side
• \e[62m: ideogram overline or left side line
• \e[63m: ideogram double overline or double line on the left side
• \e[64m: ideogram stress marking

I tested those flags in various terminal emulators:

Terminal emulator	\e[3m	\e[6m	\e[9m	\e[21m	\e[53m
Windows Terminal	✅ Yes	✅ Yes	✅ Yes	✅ Yes	✅ Yes
GNOME Terminal	✅ Yes	✅ Yes	✅ Yes	✅ Yes	✅ Yes
Konsole	✅ Yes	❌ No	✅ Yes	✅ Yes	✅ Yes
Visual Studio Code	✅ Yes	❌ No	✅ Yes	✅ Yes	✅ Yes
kitty	✅ Yes	❌ No	✅ Yes	✅ Yes	❌ No
xterm	✅ Yes	❌ No	✅ Yes	✅ Yes	❌ No
Alacritty	✅ Yes	❌ No	✅ Yes	❌ No	❌ No
iTerm2	✅ Yes	❌ No	✅ Yes	❌ No	❌ No
Termux	✅ Yes	❌ No	✅ Yes	❌ No	❌ No
Terminal.app	✅ Yes	❌ No	❌ No	❌ No	❌ No
Linux virtual console	✅ Yes	❌ No	❌ No	✅ Yes	❌ No

(The Linux virtual console does not actually render those flags as such, instead it merely recolors text. Also it supports \e[12m (second alternative font) for some reason.)

The flags not shown in the table are not supported by any of these emulators, so I assume they are truly too obscure. For the flags that are shown we should define additional values in Colorize::Mode:

module Colorize
  enum Mode
    # ...
    Italic          # 3
    BlinkFast       # 6
    Strikethrough   # 9
    DoubleUnderline # 21
    Overline        # 53
  end
end