Add optional standalone executable directory (#2)

* add standalone executable

* update readme

* add standalone workflow

* fix path

standalone/source/main.cpp

@@ -0,0 +1,46 @@
+#include <greeter.h>
+
+#include <cxxopts.hpp>
+#include <iostream>
+#include <string>
+#include <unordered_map>
+
+const std::unordered_map<std::string, greeter::LanguageCode> languages{
+    {"en", greeter::LanguageCode::EN},
+    {"de", greeter::LanguageCode::DE},
+    {"es", greeter::LanguageCode::ES},
+    {"fr", greeter::LanguageCode::FR},
+};
+
+int main(int argc, char** argv) {
+  cxxopts::Options options(argv[0], "A program to welcome the world!");
+
+  std::string language;
+  std::string name;
+
+  // clang-format off
+  options.add_options()
+    ("h,help", "Show help")
+    ("n,name", "Name to greet", cxxopts::value(name)->default_value("World"))
+    ("l,lang", "Language code to use", cxxopts::value(language)->default_value("en"))
+  ;
+  // clang-format on
+
+  auto result = options.parse(argc, argv);
+
+  if (result["help"].as<bool>()) {
+    std::cout << options.help() << std::endl;
+    return 0;
+  }
+
+  auto langIt = languages.find(language);
+  if (langIt == languages.end()) {
+    std::cout << "unknown language code: " << language << std::endl;
+    return 1;
+  }
+
+  greeter::Greeter greeter(name);
+  std::cout << greeter.greet(langIt->second) << std::endl;
+
+  return 0;
+}