设置localStorage过期时间

解决方法

自定义两个方法，存和取

###存k v
  set($k,$v){
  var expireTime = n * 1000; // 过期时间为n秒
  localStorage.setItem($k,$v)
  localStorage.setItem("key-expire", ((new Date()).valueOf()+expireTime))
},
###取k
get($k){
  if (localStorage.getItem("key-expire")< (new Date()).valueOf()) {
    localStorage.clear();
    return "";
  }else{
    return localStorage.getItem($k);
  }
}