Simplify bumping Add-on version

Bumping an Add-on version is a boring, tedious and sometimes quite error-prone routine task:
We either have to edit version_id and version_string in addon.json, pass at least option version-id on the commandline or interactively enter it in the console.

version_id is great for automatic processing, but somewhat hard to read (and way too easy to mess up) for humans.
In most cases, version changes are simple and and predictable, so couldn't this process be...