Motivation
The idea is that we rarely want to use multiple CI systems in conjunction so if we usethis ci for one e.g. bitbucket it should be possible to automatically remove any others e.g. github.

In fact, this could possibly the default behaviour, and then we'd have a --multi flag to allow multiple configs to coexist.

To do this, we should implement a CIConfig class analogous to the Tool class to include checks like is_used, which would allow us to have heuristics to determine whether other CI systems are in use, so as to delete them.

Summary of desired feature
Some opt-in or opt-out behaviour to remove any other CI configs when choosing one of them with usethis ci interfaces.