新增 客服消息，发送图文消息（点击跳转到图文消息页面）接口 (#94)

Author: finian

lib/api_message.js

@@ -254,6 +254,30 @@ exports.sendNews = async function (openid, articles) {
   return this.request(url, postJSON(data));
 };
 
+/**
+ * 客服消息，发送图文消息（点击跳转到图文消息页面）
+ * 详细细节 https://mp.weixin.qq.com/wiki?t=resource/res_main&id=mp1421140547
+ * Examples:
+ * ```
+ * api.sendMpNews('openid', 'mediaId');
+ * ```
+
+ * @param {String} openid 用户的openid
+ * @param {String} mediaId 图文消息媒体文件的ID
+ */
+exports.sendMpNews = async function (openid, mediaId) {
+  const { accessToken } = await this.ensureAccessToken();
+  var url = this.prefix + 'message/custom/send?access_token=' + accessToken;
+  var data = {
+    'touser': openid,
+    'msgtype':'mpnews',
+    'mpnews': {
+      'media_id': mediaId
+    }
+  };
+  return this.request(url, postJSON(data));
+};
+
  /**
  * 获取自动回复规则
  * 详细请看：<http://mp.weixin.qq.com/wiki/19/ce8afc8ae7470a0d7205322f46a02647.html>