feat: allow non-structs to be used as commands (#428)

* feat: allow non-structs to be used as commands

This small MR allows using the func-to-interface trick to implement a command (see commandFunc in kong_test.go).

This is useful e.g. for commands that have no flags or arguments of their own, but instead receive all required information via bound  parameters.

* fix: check DynamicCommand is runnable when adding

context.go

@@ -903,7 +903,7 @@ func checkMissingChildren(node *Node) error {
 	if len(missing) == 1 {
 		return fmt.Errorf("expected %s", missing[0])
 	}
-	return fmt.Errorf("expected one of %s", strings.Join(missing, ",  "))
+	return fmt.Errorf("expected one of %s", strings.Join(missing, ", "))
 }
 
 // If we're missing any positionals and they're required, return an error.