Improve how sub-functionality is configured

- [ ] Provide a unified API for (re)-configuring sub-functionality, rather than ad hoc functions like `infoview.set_autoopen`. Inspiration probably can be taken from` vim.diagnostic.config` (i.e. we probably should use a similar API).
- [ ] Unknown options provided to a config table should warn or error. Right now an unknown option is silently ignored, which allows typos (or mistakes due to backward incompatibility).