make it compile with MSVC

[PeterSommerlad]

Describe the bug

Does not yet compile with current MSVC compiler, even though it should be C++17 compatible

To Reproduce

Set up project in MSVC with sources and Tests.

Expected behavior

Compiles and Tests run OK.

[menuet]

Hi Peter
When looking at the readme in the repo, it is not clear whether scope.hpp compiles cleanly with MSVC. However thanks to @JoeyGrajciar's commits, it compiles at least with VS2017 and VS2019.
i think it would be better if the readme states clearly about the tested compilers and would encourage people (like me :-)) to try your library.
I will create a merge request where I add cmake + travis + appveyor stuff at your top-level directory and I replace the warning about MSVC compatibility with CI badges and list of tested compilers.