Add an ability to handle multiple config files

[phlptp]

Address Issue #486

Allow support for multiple config files

set_config("--config")->expected(1, 3);

will allow upto expected max config files.

[phlptp]

• update docs
• verify formatting and coverage

[codecov]

Codecov Report

Merging #494 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #494   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           12        12           
  Lines         3729      3731    +2     
=========================================
+ Hits          3729      3731    +2     

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

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[phlptp]

@henryiii I think this is ready now

[henryiii]

Looks good to me! One bool -> auto I'm not really fond of, but it's okay.