Inherit defaults for Options and Apps

[henryiii]

This reworks default value inheritance, as outlined in #47 . Key features:

• "Help" boolean for subcommand and app removed, can be replaced by an empty call to set_help_flag.
• Subcommand help flag inherited from parent command, make it easier to localize or change
• Ignore case is no longer mixed between subcommand and options
• App values that can be inherited: allow_extras, prefix_command, ignore_case, fallthrough, group, and footer (and help flag info)
• Option value that can be set: group, required, take_last, and ignore_case

To set option defaults:

app.option_defauts()->required();
// All future options will be required

Remaining tasks:

• Add tests
• Add README mention/clarification

In the spirit of @lambdafu 's quest to remove localized unchangeable strings, "Hidden" has been replaced by an empty string! check_style.sh is a little more accessible, as well.

[codecov]

Codecov Report

Merging #48 into master will increase coverage by 0.08%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master    #48      +/-   ##
========================================
+ Coverage   99.91%   100%   +0.08%     
========================================
  Files           9      9              
  Lines        1198   1230      +32     
  Branches      246    248       +2     
========================================
+ Hits         1197   1230      +33     
+ Misses          1      0       -1

Impacted Files	Coverage Δ
include/CLI/App.hpp	100% <100%> (+0.15%)	⬆️
include/CLI/Option.hpp	100% <100%> (ø)	⬆️

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