Add support for std::optional if macro '__cpp_if_constexpr' returns 1

[ferkulat]

I really like the idea to put command line options into optional values.
This way the parser is not responsible for setting default values.
And if one compiles with C++17 this PR would make it possible to deal with std::optional variables

[codecov]

Codecov Report

Merging #45 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #45   +/-   ##
=======================================
  Coverage   90.36%   90.36%           
=======================================
  Files           2        2           
  Lines         685      685           
=======================================
  Hits          619      619           
  Misses         66       66

Impacted Files	Coverage Δ
include/clara.hpp	87.67% <ø> (ø)	⬆️

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[philsquared]

I've just implemented support for optionals. I forgot about this PR, so I did it slightly differently - although it's very similar.

The main difference is I do use __has_include(<optional>) for detection, but along with __cplusplus >= 201703L. It seems to work on all GCC (and Clang) versions we currently support - and doesn't break on Windows, AFAICS (but I don't think the auto-detection kicks in there, yet).
You can also manually override, and supply alternate optional types by #defineing CLARA_CONFIG_OPTIONAL_TYPE to some (template) type, e.g. boost::optional (although I haven't tested that). It should work for any optional type that supports ! for validity and * for extracting the value. The underlying type must still be default constructible.

[horenmar]

I fixed parts of the optional support and added the extra tests.