SubTask: C++11 - 15 - variadic templates

[Sunrisepeak]

• 任务要求及规范见 C++11: Core Language Features #13

• 任务成果

  • 添加新章节: book/src/cpp11/15-xxx-xxx.md
  • 修改目录: book/src/SUMMARY.md
  • 新建练习代码: dslings/cpp11/15-xxx-xxx-x.cpp
  • 配置: dslings/cpp11/xmake.lua

• 参考: https://cppreference.com/w/cpp/language/parameter_pack.html