js给日期加月数，类似于java的calendar

so上看到的实现方式：

function addMonths(date, months) {

  var d = date.getDate();

  date.setMonth(date.getMonth() + +months);

  if (date.getDate() != d) {

    date.setDate(0);

  }

  return date;

}

// Add 12 months to 29 Feb 2016 -> 28 Feb 2017

console.log(addMonths(new Date(2016,1,29),12).toString());

// Subtract 1 month from 1 Jan 2017 -> 1 Dec 2016

console.log(addMonths(new Date(2017,0,1),-1).toString());

// Subtract 2 months from 31 Jan 2017 -> 30 Nov 2016

console.log(addMonths(new Date(2017,0,31),-2).toString());

// Add 2 months to 31 Dec 2016 -> 28 Feb 2017

console.log(addMonths(new Date(2016,11,31),2).toString());