Add `strictDecoding` option to Config

`circe-generic-extras` has [`strictDecoding`](https://github.com/circe/circe-generic-extras/blob/1c4b0b617849b0669a57651f601e5dd494f5b4df/generic-extras/src/main/scala/io/circe/generic/extras/Configuration.scala#L27) option which is missing here.