Preserve interleaved whitespace between record expression rows

[IwanKaramazow]

Allows whitespace interleaving between rows of record expressions.
It's hard to see the forest through the trees when scanning through Reason code containing record expression with rows packed on top of each other. By preserving one newline (if the user wrote n >= 1),
the code becomes much clearer to read and a pleasure to write. This also aligns user's expectations in terms of the output that Prettier produces.

Example:

let make = _children => {
  ...component,

  initialState: () => {
    posts: [],
    activeRoute: urlToRoute(ReasonReact.Router.dangerouslyGetInitialUrl()),
  },

  didMount: self => {
    let watcherID =
      ReasonReact.Router.watchUrl(url =>
        self.send(ChangeRoute(urlToRoute(url)))
      );
    self.onUnmount(() => ReasonReact.Router.unwatchUrl(watcherID));
  },

  reducer: (action, state) =>
    switch (action) {
    | ChangeRoute(activeRoute) => ReasonReact.Update({...state, activeRoute})
    | FetchCats => ReasonReact.NoUpdate
    },

  render: ({state: {posts, activeRoute}}) =>
    <div>
      <h1> <a href="/"> {ReasonReact.string("Instagram")} </a> </h1>
      {
        switch (activeRoute) {
        | Default => <Grid posts />
        | Detail(postId) => <Single posts postId />
        }
      }
    </div>,
};

[chenglou]

cc @anmonteiro good to know