Slack support

Added support for slack. Unfortunately the [slack plugin](https://github.com/jenkinsci/slack-plugin) requires some parameters and a publisher, so it needs 2 configs.
```yaml
properties:
  - slack:
      room: '#builds'
      onStart: false
      onAborted: true
      onSuccess: true
      onFailure: true
      onNotBuilt: true
      onUnstable: true
      onNormal: true
publishers:
  - slack:
      room: '#builds'
      teamDomain: 'teamname'
      authToken: 'yourauthtoken'
      buildServerUrl: '/'
```