changed main() created by "New C++ project" option

removing `using namespace std;` and usage of `endl`, replacing it with `\n`

src/window.cc

@@ -218,7 +218,7 @@ void Window::set_menu_actions() {
         return;
       }
       std::string cmakelists="cmake_minimum_required(VERSION 2.8)\n\nproject("+project_name+")\n\nset(CMAKE_CXX_FLAGS \"${CMAKE_CXX_FLAGS} -std=c++1y -Wall -Wextra -Wno-unused-parameter\")\n\nadd_executable("+project_name+" main.cpp)\n";
-      std::string cpp_main="#include <iostream>\n\nusing namespace std;\n\nint main() {\n  cout << \"Hello World!\" << endl;\n\n  return 0;\n}\n";
+      std::string cpp_main="#include <iostream>\n\nint main() {\n  std::cout << \"Hello World!\\n\";\n}\n";
       if(filesystem::write(cmakelists_path, cmakelists) && filesystem::write(cpp_main_path, cpp_main)) {
         Directories::get().open(project_path);
         notebook.open(cpp_main_path);