support shorthand for 100% test coverage check

* **Version**: v16.9.1
* **Platform**: Mac OS X 10.15.7 (19H1417)

I really like [`tap`'s `--100` flag](https://node-tap.org/docs/coverage/100/). Could we add something similar to `c8`? It would turn

```
c8 --check-coverage --lines 100 --functions 100 --branches 100 --statements 100  node test.js
```

into 


```
c8 --100 node test.js
```