Startup returns a unique_ptr instead of using a static

lib/include/aocpp/Startup.hpp

@@ -3,11 +3,23 @@
 
 #include <fstream>
 #include <iostream>
+#include <memory>
 #include <optional>
 
 namespace aocpp {
 
-auto Startup(int argc, char ** argv) -> std::istream&;
+class ConditionalDeleter {
+  bool should_delete_;
+
+public:
+  ConditionalDeleter(bool should_delete) : should_delete_ {should_delete} {}
+  
+  template <typename T>
+  auto operator()(T * pointer) -> void { if (should_delete_) { delete pointer; }}
+};
+
+auto Startup(int argc, char ** argv) -> std::unique_ptr<std::istream, ConditionalDeleter>;
+
 }
 
 #endif