feat: struct field tag should support both block and optional

[detailyang]

Hello.

Now the hcl cannot set optional block via the struct tag as below:

type Block struct {}
type Object struct {
	Block    Block    `hcl:"block,block,optional"`
}

[Martin Atkins]

Hi @detailyang,

Currently the way to specify that is to make the field have type *Block, so that gohcl can set it to nil to indicate when the nested block isn't present.

[detailyang]

@apparentlymart Thanks. It works :)