获取素材列表

{
   "type":TYPE,
   "offset":OFFSET,
   "count":COUNT
}

{
  "total_count": TOTAL_COUNT,
  "item_count": ITEM_COUNT,
  "item": [{
      "media_id": MEDIA_ID,
      "content": {
          "news_item": [{
              "title": TITLE,
              "thumb_media_id": THUMB_MEDIA_ID,
              "show_cover_pic": SHOW_COVER_PIC(0 / 1),
              "author": AUTHOR,
              "digest": DIGEST,
              "content": CONTENT,
              "url": URL,
              "content_source_url": CONTETN_SOURCE_URL
          },
          //多图文消息会在此处有多篇文章
          ]
       },
       "update_time": UPDATE_TIME
   },
   //可能有多个图文消息item结构
 ]
}
其他类型（图片、语音、视频）的返回如下：
{
  "total_count": TOTAL_COUNT,
  "item_count": ITEM_COUNT,
  "item": [{
      "media_id": MEDIA_ID,
      "name": NAME,
      "update_time": UPDATE_TIME,
      "url":URL
  },
  //可能会有多个素材
  ]
}

{"errcode":40007,"errmsg":"invalid media_id"}