Add --tag-message to support custom message templates for tags

[malnvenshorn]

This closes #185

The default behavior of git tag --annotate <version> --message <version> is preserved if tag_message is not set neither as parameter for the update command nor in the config file. The same substitutions as for commit_message can be used, e.g. --tag-message "Tagging {new_version} release".

If tag_message is an empty string a lightweight tag will be created, when using Git as VCS, instead of an annotated tag using git tag <version> instead. See https://git-scm.com/book/en/v2/Git-Basics-Tagging.

Checklist

• [x] Add tests
• [x] Run: make test fmt lint
• [x] Update documentation

[malnvenshorn]

Just experienced that test_hg_tag_message fails with make citest. I guess the cause is an old version of mercurial used in the docker image (version 4.0) while locally I'm using mercurial 6.4.5.