It would be nice if [`getFieldOptional`](https://github.com/purescript-contrib/purescript-argonaut-codecs/blob/master/src/Data/Argonaut/Decode/Combinators.purs#L20) was exported from `Data.Argonaut`. This issue depends on https://github.com/purescript-contrib/purescript-argonaut-codecs/issues/33 being merged and released.