allow customize label name

[bobzhang]

fix #2583,
part One of #2614

type t = {

  mutable hi : int
    [@bs.as "Content-Type"];
  mutable low : int
    [@bs.as "l"];
  mutable x : int;
    [@bs.as "open"]
} [@@bs.deriving abstract]
let v = t ~hi:3 ~low:2 ~x:2
let (a,b,c) = (v |. hi, v |. low, v |. x)
let ff () =
  v |. hiSet 3;
  v |. lowSet 2

generated code:

var v = {
  "Content-Type": 3,
  l: 2,
  open: 2
};

var a = v["Content-Type"];

var b = v.l;

var c = v.open;

function ff() {
  v["Content-Type"] = 3;
  v.l = 2;
  return /* () */0;
}

[chenglou]

cc @jaredly this is what deriving abstract generates