Start to prepare for help generation

examples/dune

@@ -17,3 +17,8 @@
  (names group)
  (libraries climate)
  (modules group))
+
+(executables
+ (names optional_args)
+ (libraries climate)
+ (modules optional_args))

examples/optional_args.ml

@@ -0,0 +1,10 @@
+open Climate
+
+let term =
+  let open Arg_parser in
+  let+ x = named_opt_with_default [ "x" ] string ~default:"foo"
+  and+ y = named_opt_with_default [ "y" ] string ~default:"bar" in
+  Printf.printf "%s and %s" x y
+;;
+
+let () = Command.singleton term |> Command.run