Adding enum support

[henryiii]

This fixes #12. There are two limitations to the enumeration support:

• You can't pick an enumeration by name from the command line. This is a limitation of C++, reflexion would be need to be added to C++ to support this.
• Only traditional style enums are supported, since they do not support << . This is a limitation that can be lifted at some point based on the number of arguments to the add_ command. This is now fixed; new style enums are supported too.

Help strings for enums cannot be automatically generated, however, you can set them yourself.

[codecov]

Codecov Report

Merging #15 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #15   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines        1048   1108   +60     
  Branches      220    227    +7     
=====================================
+ Hits         1048   1108   +60

Impacted Files	Coverage Δ
include/CLI/TypeTools.hpp	100% <ø> (ø)	⬆️
include/CLI/App.hpp	100% <100%> (ø)	⬆️
include/CLI/Validators.hpp	100% <0%> (ø)	⬆️

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