How do you create a decoder with an fp-ts Option field?

Hi,

How do you create a decoder with an Option type?

```
import * as D from 'io-ts/lib/Decoder'
import * as O from 'fp-ts/lib/Option'

const Response = D.array(D.struct({
    ip: D.string,
    countryName: O.Option<string> <-- ??
}))
```