start adding doctests

lib/src/Startup.cpp

@@ -5,10 +5,18 @@
 #include <optional>
 #include <memory>
 #include <fstream>
+#include <utility>
+
+#define DOCTEST_CONFIG_IMPLEMENT
+#include <doctest.h>
 
 namespace aocpp {
 
 auto Startup(int argc, char ** argv) -> std::istream& {
+  if (std::getenv("DOCTEST")) {
+    exit(doctest::Context{argc, argv}.run());
+  }
+
   static std::ifstream fin;
   switch (argc) {
   case 2: fin = std::ifstream{argv[1]}; return fin;