Implement a customization point for types not default constructible

[ozars]

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This patch adds a customization point YAML::decode_dispatcher<T>::dispatch which:

• allows decoding types that aren't default constructible,
• delegates to YAML::convert<T>::decode by default for backward compatibility,
• is enabled only when compiled with C++17 or newer since it depends on std::optional.

Notes:

• Compiling with C++17 automatically enables this feature.
• Relevant tests are added. Tested it locally (gcc 10.2 on debian buster) to confirm all tests passes with both C++11 & C++17.
• YAML_CPP_HAS_DECODE_DISPATCHER macro is exposed allowing checking this feature.
• Its signature has an Enable template parameter giving more flexibility, e.g. to allow SFINAE downstream:

namespace YAML {
template <typename T, typename Enable = void>
struct decode_dispatcher {
  static std::optional<T> dispatch(const Node& node) {
   // ...
  }
};
}

Example:

#include <yaml-cpp/node/impl.h>

struct Vec3 {
  double x, y, z;
  Vec3(double x, double y, double z) : x(x), y(y), z(z) {}
  // ...
};

namespace YAML {
template<>
struct decode_dispatcher<Vec3> {
  static std::optional<Vec3> dispatch(const Node& node) {
    if(!node.IsSequence() || node.size() != 3)
      return std::nullopt;
    return Vec3(
      node[0].as<double>(),
      node[1].as<double>(),
      node[2].as<double>()
    );
  }
};
}

Possibly closes: #993.