Change $currentVersion parameter to a Version instance.

This makes the interface a little cleaner and simpler to reason with.
Prompt for exact version with autocomplete.

Prompting for the exact version makes it easier to specify custom
version numbers to tag, including versions with build/prerelease data.

The simple behavior behind the common cases of patch/minor/major
increments are maintained [for the most part] by utilizing autocomplete
where the versions are automatically entered for the user as they type.