How to support slice of struct?

For example:
```
type confModule struct {
	Name   string   `yaml:"name" default:"abc"`
	Params []string `yaml:"params" default:"[\"-c\", \"conf.yml\"]"`
}

type confStruct struct {
	ID     string       `yaml:"id"`
	Modules []confModule `yaml:"modules"`
}
```

defaults will skip the slice of struct (confStruct.Modules), how to support this case?