Support encapsulated model structure with private field.

When using `autobind` for model, currently gqlgen only supports the model struct with public fields.

From the example: https://github.com/99designs/gqlgen/blob/master/docs/content/getting-started.md

```golang
type Todo struct {
ID string `json:"id"`
Text string `json:"text"`
Done bool `json:"done"`
UserID string `json:"userId"`
User *User `json:"user"`
}
```

But, how about encapsulated model like,

```golang
type Todo struct {
id string
text string
done bool
userID string
user *User
}
```

I want forbid direct access and update of model field to ensure immutability of the model. And, only external function can handle that. In this case, can we support this model for gqlgen too?