Uptime Slack webhook plugin

This Uptime plugin notifies all configured events (up, down, paused, restarted) by sending a HTTPS POST request to a slack.com URL.

Based on mintbridge/uptime-webhooks plugin.

To use this plugin, first install it using npm while in the Uptime directory:

$ npm install uptime-slack

Then to enable it, add it to the plugins/index.js, as follows:

// in plugins/index.js
exports.init = function() {
  require('uptime-slack').init();
}

Customize the plugin settings in the config/production.yaml configuration file, as in the example below:

webhooks:
  event:
    up:
      - 'https://xxxx.slack.com/services/hooks/incoming-webhook?token=xxxxxx'
    down:
      - 'https://xxxx.slack.com/services/hooks/incoming-webhook?token=xxxxxx'
    paused:
      - 'https://xxxx.slack.com/services/hooks/incoming-webhook?token=xxxxxx'
    restarted:
      - 'https://xxxx.slack.com/services/hooks/incoming-webhook?token=xxxxxx'
  dashboardUrl: 'http://uptime.example.com'
  channel:      '#slack-channel'
  username:     'uptime'
  icon_emoji:   ':fire:'