Support DisallowUnknownFields in golang 1.10

[evie404]

Hi,

Thanks a lot for creating this package. It has been very helpful in my project. One small request:
json package in golang 1.10 will introduce DisallowUnknownFields() which would throw an error if a field is missing during unmarshaling. it would be great if json-iterator can support the same configuration.

Reference change in golang:
https://go-review.googlesource.com/c/go/+/74830

Thank you for considering this request!

[taowen]

it is supported now