Add support to update messages #75

[kuboon]

Summary

• add test
• add usage doc

#75

• outputs.time was not what I need. Added outputs.ts
• added update-ts option which calls chat.update api

- id: slack
  uses: slackapi/slack-github-action@v1.18.0
  with:
    channel-id: "#channel"
    payload: |
      {
        "attachments": [
          {
            "pretext": "Deployment started",
            "color": "dbab09",
            "fields": [
              {
                "title": "Status",
                "short": true,
                "value": "In Progress"
              }
            ]
          }
        ]
      }
  env:
    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN}}
- uses: slackapi/slack-github-action@v1.18.0
  with:
    channel-id: "#channel"
    update-ts: ${{ steps.slack.outputs.ts }}
    payload: |
      {
        "attachments": [
          {
            "pretext": "Deployment finished",
            "color": "28a745",
            "fields": [
              {
                "title": "Status",
                "short": true,
                "value": "Completed"
              }
            ]
          }
        ]
      }
  env:
    SLACK_BOT_TOKEN: ${{ secrets.SLACK_BOT_TOKEN}}

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[CLAassistant]

All committers have signed the CLA.

[seratch]

@kuboon Thanks a lot for taking the time to work on this feature addition! Once my suggestions on the README changes are resolved, the change looks great to me.

I would like to know what @stevengill @filmaj think about this feature addition (refer to #75 for context) and review this PR!

[seratch]

LGTM; Still waiting for others' reviews.

[stevengill]

LGTM! Thanks!