Moving error messages definitions to Error.h

[henryiii]

This cleans up the rest of the message to make localization easier (still not explicitly included, but easier when most of the localized text is in one place!). This probably is the last PR to 1.3.

This also gives options an inheritable ->configurable(false) setting, to allow the user to exclude some options from the ini file.

The add config has been renamed to set_config, and calling it with no arguments clears the config option.

[codecov]

Codecov Report

Merging #60 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #60   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines        1296   1342   +46     
  Branches      248    251    +3     
=====================================
+ Hits         1296   1342   +46

Impacted Files	Coverage Δ
include/CLI/Ini.hpp	100% <100%> (ø)	⬆️
include/CLI/Error.hpp	100% <100%> (ø)	⬆️
include/CLI/App.hpp	100% <100%> (ø)	⬆️
include/CLI/Split.hpp	100% <100%> (ø)	⬆️
include/CLI/Option.hpp	100% <100%> (ø)	⬆️

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