Alerters: support wechat.

[daiwei233]

Wechat is a Chinese enterprise IM, offical website: https://open.work.weixin.qq.com/api/doc/90000/90135/90664.

Just like the alertmanager: https://prometheus.io/docs/alerting/latest/configuration/#wechat_config

[nsano-rururu]

We are waiting for your pull request.

[nsano-rururu]

I don't use wechat so I can't check the operation.
Please refer to the following to make a pull request.

https://github.com/Zhang21/elastalert-wechatrobot-plugin

[daiwei233]

okay.

[nsano-rururu]

The files to modify are loaders.py and schema.yaml.
The files to add are wechat.py and wechat_test.py.
Please note that the source code structure is different from yelp elastalert.

elastalert
  loaders.py
  schema.yaml
  /alerters
    wechat.py
  /tests
    /alerters
      wechat_test.py

[nsano-rururu]

Added alert name to elastalert.rst. Added description about setting wechat to ruletypes.rst.

docs/source/
  elastalert.rst
  ruletypes.rst

[daiwei233]

Hi, i have made a pull request: #355