Support colored help output

[Zhou-London]

This PR adds support for the colored help output regarding the dicuss in issue: #1285
Functions are added into the Default Formatter. When color_enabled_ is true, ANSI codes are injected into the stream to display various colors.

Certain formatting logic has been modified in the colored branch (color_enabled_ == true):

• Count visual_length instead of the actual length, for that the latter one include the ANSI codes, which are not displayed.
• Colorize the header/option/etc. before putting them to the stream.
• The color palette is defined as macros in Macros.h

Usage:

app.get_formatter()->enable_color();

An example and a new test has been added. Readme has been updated as well. All tests are passed on my x86 Linux (Arch)
machine.

[codecov]

Codecov Report

❌ Patch coverage is 75.17730% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 99.33%. Comparing base (e4ee3af) to head (ca9afcc).
⚠️ Report is 195 commits behind head on main.

Files with missing lines	Patch %	Lines
include/CLI/impl/Formatter_inl.hpp	81.10%	24 Missing ⚠️
include/CLI/impl/StringTools_inl.hpp	0.00%	11 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main    #1328      +/-   ##
===========================================
- Coverage   100.00%   99.33%   -0.67%     
===========================================
  Files           17       19       +2     
  Lines         4546     5243     +697     
  Branches         0     1121    +1121     
===========================================
+ Hits          4546     5208     +662     
- Misses           0       35      +35     

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[Sckab]

please don't quit this PR, it's so good and helpful