Add a way to decode `Value` into arbitrary Elm value

[niho]

Problem

The validated output from a form is represented as a Value. It is easy to encode such a Value into JSON using the Json.Schema.From.Encode module. But if you instead want to use the value in your Elm program it becomes a bit more cumbersome to use the generic Value type.

Solution

Create a way to write decoders that can turn a Value into an arbitrary Elm value. It would work the same exact way as a a Json.Decode decoder.

A nice bonus would be if it was possible to also include metadata from the schema when decoding, so that you can annotate your Elm value with for example the title attribute from the schema.