Rule to separate fields by newline

Hi, would it be easy to add a rule that would disallow this:

```
message Foo {
  // Example 1
  int32 field_one = 1;
  // Example 2
  int32 field_two = 2;
}
```

and force you to do this:

```
message Foo {
  // Example 1
  int32 field_one = 1;

  // Example 2
  int32 field_two = 2;
}
```