* Add rough draft of `check` mode. Not unit tested.
* Added assert-cli; simple test case for `--write-mode=check`
* Lightly documented `check` WriteMode
* wrote clearer phrasing for config::options::WriteMode::Check
* Implemented default for WriteMode where default is Overwrite
* Simplified exit code handling
* updated README.md as per @nrc' comment
* collapsed exit-code handling
* Removed write_mode from Summary, introduced partial option parsing earlier
* Handle write-mode parsing in a slightly better way.
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