Support for JSON encoders/decoders.

[mplzik]

Proto3 supports canonical encoding in JSON, implementing JSON-based encoders and decoders would allow to easily plug elm in front of backends that use proto3 for API description. tiziano88/elm-protobuf that was implementing this, but unfortunately it doesn't seem to be maintained anymore and their protoc-gen-elm plugin doesn't generate valid code anymore.

[eriktim]

Hi @mplzik, I am not actively adding new features to this package. Feel free to create a PR if you want.