Slack's payload validation not implemented

[ryysud]

The template for slack's payload was wrong and failed to execute workflow as follows.

$ cat sample.dig
_export:
  plugin:
    repositories:
      - https://jitpack.io
    dependencies:
      - com.github.szyn:digdag-slack:0.1.4
  webhook_url: https://hooks.slack.com/services/xxx
  workflow_name: slack
  ENV: develop

+first_task:
  echo>: "run first task!"
  _check:
    slack>: templates/success.yml

+second_task:
  echo>: "run other tasks!"

$ cat templates/success.yml
---
username: Digdag
icon_emoji: ':blush:'
channel: '#digdag-slack-test'
typo_attachments: <----------------------------------- typo :(
- fallback: '[SUCCESS] ${workflow_name} workflow'
  color: "good"
  text: '*[SUCCESS]* `${workflow_name}` Workflow'
  mrkdwn_in:
  - text
  - pretext
  - fields
  fields:
  - title: Task Name
    value: "${task_name}"
    short: false
  - title: Session Date
    value: "${session_date}"
    short: true
  - title: Environment
    value: "${ENV}"
    short: true

$ digdag run -a sample.dig
2017-11-30 05:47:58 +0000: Digdag v0.9.21
2017-11-30 05:48:01 +0000 [WARN] (main): Reusing the last session time 2017-11-30T00:00:00+00:00.
2017-11-30 05:48:01 +0000 [INFO] (main): Using session /root/digdag-slack/.circleci/.digdag/status/20171130T000000+0000.
2017-11-30 05:48:01 +0000 [INFO] (main): Starting a new session project id=1 workflow name=sample session_time=2017-11-30T00:00:00+00:00
2017-11-30 05:48:04 +0000 [INFO] (0016@[0:default]+sample+first_task): echo>: run first task!
run first task!
2017-11-30 05:48:06 +0000 [INFO] (0016@[0:default]+sample+first_task^check): slack>: templates/success.yml
java.io.IOException: posting to slack failed
	at io.digdag.plugin.slack.SlackOperatorFactory$SlackOperator.postToSlack(SlackOperatorFactory.java:89)
	at io.digdag.plugin.slack.SlackOperatorFactory$SlackOperator.runTask(SlackOperatorFactory.java:69)
	at io.digdag.util.BaseOperator.run(BaseOperator.java:35)
	at io.digdag.core.agent.OperatorManager.callExecutor(OperatorManager.java:312)
	at io.digdag.cli.Run$OperatorManagerWithSkip.callExecutor(Run.java:694)
	at io.digdag.core.agent.OperatorManager.runWithWorkspace(OperatorManager.java:254)
	at io.digdag.core.agent.OperatorManager.lambda$runWithHeartbeat$2(OperatorManager.java:137)
	at io.digdag.core.agent.LocalWorkspaceManager.withExtractedArchive(LocalWorkspaceManager.java:25)
	at io.digdag.core.agent.OperatorManager.runWithHeartbeat(OperatorManager.java:135)
	at io.digdag.core.agent.OperatorManager.run(OperatorManager.java:119)
	at io.digdag.cli.Run$OperatorManagerWithSkip.run(Run.java:676)
	at io.digdag.core.agent.MultiThreadAgent.lambda$null$0(MultiThreadAgent.java:127)
	at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:511)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
2017-11-30 05:48:10 +0000 [INFO] (0016@[0:default]+sample+second_task): echo>: run other tasks!
run other tasks!
Success. Task state is saved at /root/digdag-slack/.circleci/.digdag/status/20171130T000000+0000 directory.
  * Use --session <daily | hourly | "yyyy-MM-dd[ HH:mm:ss]"> to not reuse the last session time.
  * Use --rerun, --start +NAME, or --goal +NAME argument to rerun skipped tasks.

I tried to find typo, but it was difficult to notice typo.
Do you have any plans to implement slack's payload validation according to slack's official format?
https://api.slack.com/docs/messages/builder