Support rejecting unknown key feature

I think it would be nice to support the rejecting unknown key feature like [DisallowUnknownFields()](https://golang.org/pkg/encoding/json/#Decoder.DisallowUnknownFields) of encoding/json as below:

```go
err := hcl.hasUnknownKey(structure, input)
```

If you don't plan to support this feature, I would like to know if there is a way to reject unknown key using current features.

Start by reviewing the HCL decoding API and the linked encoding/json Decoder.DisallowUnknownFields behavior. Determine whether the requested hcl.hasUnknownKey(structure, input) behavior can be defined consistently, including how unknown keys should be reported; the issue is complete when the intended API or current-feature workaround is documented and verified.

Written by the indexing model from the issue text.