Ability to set placeholder for future versions

[fizyk]

I'd like to have the ability to set a placeholder for future versions (for documentation), that would be replaced by tbump if found, and not rised an exception if not.

The idea is to allow for documenting features added in specific new versions, or deprecated since versions like adding thse directives to docstring and/or project documentation: https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#describing-changes-between-versions