title: elastalert configure slack notification
body: <p>hi trying configure slack notification in test.yaml file , getting the below error ERROR:root:Could not load rule /opt/rules/test.yaml: Error initiating alert ['slack', {'slack_webhook_url': '<a href="https://hooks.slack.com/servichttps://hooks.slack.com/services/abcd" rel="nofollow">https://hooks.slack.com/servichttps://hooks.slack.com/services/abcd</a>'}]: Could not import module slack_webhook_url: need more than 1 value to unpack
  alert:
  "slack"
  slack_webhook_url: "<a href="https://hooks.slack.com/servichttps://hooks.slack.com/services/abcd" rel="nofollow">https://hooks.slack.com/servichttps://hooks.slack.com/services/abcd</a>"
  is this the correct way to define the slack alert</p>