docs: add TODO.md for parser callback feature

TODO.md

@@ -0,0 +1,22 @@
+# TODO 1: After Parse callback
+Currently interdependent arguments are a problem. In the current version we are handling them as closures. 
+We create two stateful actions who are taking the state from the close over of the lambda. This is a little hacky/ugly. 
+To fix this we could have a callback/on_parse_done event. An example usage could be; 
+```cpp
+parser.after_parse([&parser]() {
+    auto grep_val = parser.get_optional<std::string>("grep");
+    auto file_val = parser.get_optional<std::string>("file");
+    
+    if (grep_val && file_val) {
+        grep(*grep_val, *file_val); 
+    } else if (grep_val) {
+        std::cerr << "grep requires file\n";
+    }
+});
+```
+
+After handle_parse(...) ends, we can call the on_parse_done callbacks. But this requires two things; 
+1. Storable arguments. Those would be not actions, but stored values. So you could get them later on in the callback or somewhere else.
+2. The on_parse_done callbacks should be called after all actions are invoked. But we should make this a toggle, so the callback only gets executed if the user explicitly wants it to be called. By default we would enable it. 
+
+## Self reminder, implement this feature. It'd be helpful.