add an alias section to the help for subcommands

[phlptp]

Address issue #448

In most cases the aliases will show as

sub1, sub_alias1, sub_alias2 Description

in a few cases of option groups with aliases it will show as

aliases: sub_alias1, sub_alias2

[codecov]

Codecov Report

Merging #545 (1284f2d) into master (28b35af) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #545   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines         3766      3789   +23     
=========================================
+ Hits          3766      3789   +23     

Impacted Files	Coverage Δ
include/CLI/App.hpp	100.00% <100.00%> (ø)
include/CLI/Formatter.hpp	100.00% <100.00%> (ø)
include/CLI/StringTools.hpp	100.00% <100.00%> (ø)

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