Added MSVC checking for support for string_view.

[mwinterb]

Microsoft allows customizing the standard version the compiler will target, so a simple _MSC_VER check is inadequate, but __has_include support was added after string_view, so _MSC_VER is still needed.

Also, adding "override" to destructors that are virtual by inheritance.

[vitaut]

Looks great & thanks for the PR!
One small request: could you split the long line for readability?

[mwinterb]

I've fixed that, but I've noticed that the CStringRefTest.Ctor test does not compile with MSVC now. https://github.com/fmtlib/fmt/blob/master/test/format-test.cc#L177
But I don't think that it should compile regardless? If it is compiling, it would appear it's converting from:
string_view -> string -> CStringRef. But the string_view -> string conversion is supposed to be explicit:
string.cons.
In fact, both gcc and clang also do not compile this: https://godbolt.org/g/dEVCQV.

[vitaut]

Merged, thanks!