Support unions in elm

[mbryzek]

Create json decoders for Elm unions:

type SessionState =
    SessionStateCommunitySession CommunitySession
    | SessionStateUserInactive UserInactive

sessionStateDecoder : Decode.Decoder SessionState
sessionStateDecoder =
    Decode.field "discriminator" Decode.string
        |> Decode.andThen (\disc ->
            case disc of
                "community_session" ->
                    communitySessionDecoder |> Decode.map SessionStateCommunitySession

                "user_inactive" ->
                    userInactiveDecoder |> Decode.map SessionStateUserInactive

                _ ->
                    Decode.fail ("Unknown discriminator: " ++ disc)
        )