Adding simple flag callback

[henryiii]

This should fix #32 in a general way, I think.

.add_flag_functional("--name", [](size_t count){...});

C++14 shortcut:

.add_flag("--name", [](size_t count){...});

This can't be added to C++11 since the compiler can't tell the difference between std::string and std::function<void(size_t)> for a const char * argument until C++14 (or C++11 mode on a C++14 compiler, since this was back-fixed as a bug, but that's too specific to provide #if's for).

[codecov]

Codecov Report

Merging #33 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #33   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines        1121   1141   +20     
  Branches      231    233    +2     
=====================================
+ Hits         1121   1141   +20

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

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[infinity0n3]

I can confirm that it works now. Would have been nicer to have the add_flag syntax but I have to use c++11.
Good thing is that it's a one liner now.

Thank you!