ghc::filesystem Release v1.2.0

Added MingW 32/64 and Visual Studio 2015 builds to the CI configuration.
Fixed additional compilation issues on MingW.
Pull request (#13), set minimum required CMake version to 3.7.2 (as in Debian 8).
Pull request (#14), added support for a make install target.
Bugfix for (#15), the forward/impl way of using ghc::filesystem missed a <vector> include in the windows case.
Bugfix for (#16), VS2019 didn't like the old size dispatching in the utf8 decoder, so it was changed to a sfinae based approach.
New feature (#17), optional support for standard conforming wchar_t/std::wstring interface when compiling on Windows with defined GHC_WIN_WSTRING_STRING_TYPE, this is default when using the ghc/fs_std*.hpp header, to enhance compatibility.
New feature (#18), optional filesystem exceptions/errors on unicode errors with defined GHC_RAISE_UNICODE_ERRORS (instead of replacing invalid code points or UTF-8 encoding errors with the replacement character U+FFFD).
Pull request (#20), fix for file handle leak in fs::copy_file.
Coverage now checked in CI (~95% line coverage).

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