* 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.
For example, with the following config file, rustfmt will ignore `src/types.rs`,
`src/foo/bar.rs` and every file under `examples/` directory.
```toml
[ignore]
files = [
"src/types.rs",
"src/foo/bar.rs",
]
directories = [
"examples",
]
```
