Ability to skip over _ prefixed fields

How big of a task is it to skip over these fields when deriving Serialize?

Ex: 
```rs
struct Ye {
    a: u16,
    _b: u8,
    c: u32,
}
```
should produce

```json
{ "Ye":
  { 
    "a": 0,
    "c": 0
  }
}
```