Provide a way to override generic encoding/decoding for certain data types

[eskimor]

For encoding Nothing as null for example, or Tuple a b as [a, b], ....
We could include options in Data.Argonaut.Options.Options, called userEncoding, userDecoding:

userEncoding :: GenericSignature -> GenericSpine -> Maybe Json
userDecoding :: GenericSignature -> Json -> Maybe GenericSpine

or something like this. genericEncodeJson'/genericDecodeJson' would then try those functions first on each recursion. Thus, the user can choose to override the encoding/decoding for certain signatures.

[eskimor]

WIP.

[eskimor]

use purescript-argonaut-generic-codecs for this.