added crow

include/crow/returnable.h

@@ -0,0 +1,19 @@
+#pragma once
+
+#include <string>
+
+namespace crow
+{
+    /// An abstract class that allows any other class to be returned by a handler.
+    struct returnable
+    {
+        std::string content_type;
+        virtual std::string dump() const = 0;
+
+        returnable(std::string ctype):
+          content_type{ctype}
+        {}
+
+        virtual ~returnable(){}
+    };
+} // namespace crow