fix: add --tag-only flag to tag HEAD without bumping

[maimul]

Problem

There's no way to tag the current HEAD using the version from config without also bumping version numbers. This blocks a common workflow where a build artifact needs to be committed between the version bump commit and the release tag:

bumpversion patch         # bumps 1.0.0 -> 1.0.1, commits, but also tags too early
# ... build artifact, can't add it to the tag commit after the fact

Fix

Add --tag-only flag. When passed, bumpversion reads current_version from config, tags HEAD, and exits — no file edits, no commit.

Usage

bumpversion patch --no-tag     # bump + commit, skip the tag
# ... build artifact, git add, git commit ...
bumpversion --tag-only         # tag HEAD now at the bumped version

The --tag-only flag is parsed in phase 1 (before any positional argument is required), so no patch/minor/major argument is needed.

Fixes #256