Feature: tag message from config (#453)

[ebAtUelzener]

This PR implements a way to set the tag message via a callback.

It does not change default behaviour of the plugin.

The explicit suggestion from the original issue (relaying the commit message) is not intended to be implemented by this, however it is possible to provide extra information from CI using e.g. Gradle properties/ environment variables and reading these to set the message.

Minimal example reading an environment variable:

scmVersion {
  tag {
    messageCreator { position, version, previousVersion ->
      val envVar = System.getenv("SOME_VAR")
      "This is a message on version $version (previous: $previousVersion) {commit: ${position.shortRevision}} [$envVar]"
    }
  }
}

This would allow users to use arbitrary logic to set a tag message, without changing too much of the plugin's internals.