feat: add quiz 2 counter.

counter/main.cpp

@@ -0,0 +1,21 @@
+#include <counter.h>
+#include <iostream>
+
+int main(const int argc, char** argv)
+{
+    if (argc != 2)
+    {
+        std::cout << "ERROR: Need a string." << std::endl;
+    }
+
+    const std::string input(argv[1]);
+
+    const auto map = counter::Counter::count(input);
+
+    for (const auto& pair : map)
+    {
+        std::cout << pair.first << ' ' << pair.second << std::endl;
+    }
+
+    return 0;
+}