Support Decoder options for Unmarshal

[FeldrinH]

It would be nice if this library could support Decoder.UseNumber and Decoder.DisallowUnknownFields on the Unmarshal method as well. Currently, as far as I can tell, using these options requires using Decoder, which has two disadvantages:

• If you have data that is already in a byte array then Decoder is slower according to the benchmarks
• As far as I know Decoder has no reliable method to ensure that there is no extra data after the read json object (so for example {"value": 323};,fdaslfdaksf is considered valid by the decoder, if you only decode one object).

[kaptinlin]

any plan support DisallowUnknownFields?

[zhujunping-zjp]

Is there any plan to support Decoder.UseNumber on the Unmarshal method.