Allow custom Slack username and icon

Add a couple of optional settings for customising the reported user and icon:

  * `JANKY_CHAT_SLACK_USERNAME` is the username to report as (default: `hubot`)
  * `JANKY_CHAT_SLACK_ICON_URL` is the icon to display (default: [labtocat][])

[labtocat]: https://octodex.github.com/images/labtocat.png