Allow short_commit_id in subject template

I've been using git-commit-notifier with

    subject: "[${prefix}${slash_branch_name}] ${description} - ${message}"

happy that the description is always 7-chars long, which makes the messages align nicely in my Mutt, and then I went and started using annotated tags, which makes git describe use needlessly long descriptions that also vary in length.

I'd like to get my nice short abbreviated commit hashes back, without having to stop using annotated tags.  So I hacked up support for

    subject: "[${prefix}${slash_branch_name}] ${short_commit_id} - ${message}"

(thank you very much for [telling me how to test local changes](https://github.com/git-commit-notifier/git-commit-notifier/issues/173#issuecomment-14117976))