Customize when Slackbot posts releases

[kharrop]

Is your feature request related to a problem? Please describe.

Depending on the frequency of releases, the Slack plugin could potentially be a bit too spammy - especially if folks have their repos already set to pull in patch/minor updates.

Describe the solution you'd like

Be able to specify if the bot should announce a release based on the type. For example, this would only trigger the bot in a channel for major releases only:

{
  "plugins": [
    [
      "slack",
      {
        "releaseType": "major",
      }
    ]
  ]
}

Describe alternatives you've considered

Additional context