Custom Slack Notification Message

[clifforous]

Would be nice to add a custom message to a notification, maybe per event.
Something like

notify:
  slack:
    channel: "#ad_ci"
    identifier: "L11"
    on:
      - event: "FAIL"
        message: "@Cliff your build failed"

[tjtelan]

Another add to this request is that we allow some way to pass dynamic information to the message from the build.

Possibly even the output of a script that is provided in the slack block, or somewhere else relative to the repo?