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

build.go

@@ -51,6 +51,9 @@ type flattenedField struct {
 
 func flattenedFields(v reflect.Value, ptag *Tag) (out []flattenedField, err error) {
 	v = reflect.Indirect(v)
+	if v.Kind() != reflect.Struct {
+		return out, nil
+	}
 	for i := 0; i < v.NumField(); i++ {
 		ft := v.Type().Field(i)
 		fv := v.Field(i)