Make caching work more robust for third-party style guides

[lorenzwalthert]

Transformers should have a version, so we don't need to cache based on text of transformers (as this won't work in general for third party style guides) and revise documentation for it. This affects the mlr style guide for example. WIP at https://github.com/lorenzwalthert/styler/tree/issue-571.