Improve `--check-cfg` implementation
This pull-request is a mix of improvements regarding the `--check-cfg` implementation:
- Simpler internal representation (usage of `Option` instead of separate bool)
- Add --check-cfg to the unstable book (based on the RFC)
- Improved diagnostics:
* List possible values when the value is unexpected
* Suggest if possible a name or value that is similar
- Add more tests (well known names, mix of combinations, ...)
r? ```@petrochenkov```
000e38d9cb