Feature Request: minimum blank lines between fields with comments

This can make structures with big field documentation more readable.

e.g.:
`0`:
```rust
struct Tets {
    /// Doc
    a: i32,
    // Comment
    b: i32,
    c: i32,
    /// Doc2
    d: i32,
}
```
`1`:
```rust
struct Tets {
    /// Doc
    a: i32,

    // Comment
    b: i32,
    c: i32,

    /// Doc2
    d: i32,
}
```
