Support DisallowUnknownKeys option

[ansel1]

Not crazy about the name, but essentially an option like gorilla/schema's IgnoreUnknownKeys option. Useful for really strict validation of incoming query params.

[deankarn]

💡 I like the idea @ansel1

I’ll have to think about how to implement as this library sets the data from the structs perspective and not the incoming data, but I’ll figure something out

[ansel1]

maybe add a slice or map to decoder to track used keys, then compare to original values at the end of Decoder.Decode()