Add version information

[mxmlnkn]

It might be helpful to have version information defined in the header, e.g., cxxopts has something like this:

#define CXXOPTS__VERSION_MAJOR 3
#define CXXOPTS__VERSION_MINOR 1
#define CXXOPTS__VERSION_PATCH 1

namespace cxxopts {
static constexpr struct {
  uint8_t major, minor, patch;
} version = {
  CXXOPTS__VERSION_MAJOR,
  CXXOPTS__VERSION_MINOR,
  CXXOPTS__VERSION_PATCH
};
} // namespace cxxopts