Add support for metadata in gluamapper.Option struct

[dnaeon]

Adding support for metadata allows for inspecting the result of decoding and is useful in situations where we need to verify a list of required fields or unused ones.

A better approach would be to simply embed mapstructure.DecoderConfig in gluamapper.Option, but doing that would introduce a breaking API change, so with this commit I'm simply adding mapstructure.Metadata to gluamapper.Option struct for now. In future commits we should probably consider simply embedding DecoderConfig into Option struct.

[yuin]

Thanks for you PR, and sorry for my late reply.(My job has gotten a little busy)

I think that it would be better to hide backend library APIs from gluermapper APIs.

Adding something like the following code to gluamapper.go

type Metadata struct {
    Keys []string
    Unused []string
}

    metadata := &mapstructure.Metadata{}
    metadata.Keys = opt.Metadata.Keys
    metadata.Unused = opt.Metadata.Unused 
    config := &mapstructure.DecoderConfig{
        WeaklyTypedInput: true,
        Result:           st,
        TagName:          opt.TagName,
        ErrorUnused:      opt.ErrorUnused,
        Metadata:         metadata,
    }

Are you in favor of this plan?

[dnaeon]

Sounds good. Do you want me to update the PR or you will do it instead?

[yuin]

I would appreciate it if you could update the PR.