Customize when Slackbot posts releases

**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:

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

**Describe alternatives you've considered**

**Additional context**