Please consider supporting struct-based documents

Right now, this library supports interface-based documents of type `map[string]interface{}` and `[]interface{}`. It would be useful to support any arbitrary `struct`-based or `map[Stringer]anything`-based document supported by `encoding/json` such as:
```go
type Document struct {
    Foo Bar
}

type Bar struct {
    Number float64
}
```
With the pointer:
`/Foo/Number`