feat: allow build system to be set to C++17, rename multithread var

[henryiii]

Fix #501

Support setting C++ mode in setuptools or CMake. Use the CMake naming for parallel threading. CPython 3.8 compiles in C++17 mode.

Not bothering to fix Travis because I'll be dumping it in #474 and want to avoid the merge conflict. The Conda recipe will need an update, though.

[henryiii]

Weird. Fixed, anyway, but might be an issue in pybind11, it should not be mixing a class and a subclass, especially only on C++17. Reproduced locally with Clang, as well.

[henryiii]

The issue is pybind11 doesn't support binding noexcept methods in C++17 mode. The lambda is the correct workaround for now until someone fills out the combinatorics of adding this, or finds a better way to do it.