feat: added usage message replacement feature

[polistern]

Usage message replacement

Now you able to set custom usage message or pass callback function that will generate it for you.
Example:

CLI::App app{"app v1"};
app.usage("use: [-h|--help] | --option UINT");

Output will be:

app v1
use: [-h|--help] | --option UINT

Options:
  -h,--help                             Show this message

To void usage message replacement in subcommand and be able to get generated usage message just set with empty string:

auto sub = app.add_subcommand("sub");
sub->usage("");

Help message for app -h:

app v1
use: [-h|--help] | --option UINT

Options:
  -h,--help                             Show this message

Subcommands:
sub

Help message for app sub -h:

Usage: app sub [OPTIONS]

Options:
  -h,--help                             Show this message

Footer newline behavior

Moved to #796

[codecov]

Codecov Report

Base: 99.42% // Head: 99.42% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (a6bf0aa) compared to base (c2ea58c).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #786   +/-   ##
=======================================
  Coverage   99.42%   99.42%           
=======================================
  Files          16       16           
  Lines        3977     3989   +12     
=======================================
+ Hits         3954     3966   +12     
  Misses         23       23           

Impacted Files	Coverage Δ
include/CLI/App.hpp	100.00% <100.00%> (ø)
include/CLI/impl/App_inl.hpp	99.20% <100.00%> (+<0.01%)	⬆️
include/CLI/impl/Formatter_inl.hpp	99.41% <100.00%> (+0.01%)	⬆️

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[henryiii]

Thanks! Moving on with 2.4.