Instead of using:
`coverage: /\(\d+.\d+%\) covered/`

This structure must be used now:
```
coverage:
  output_filter: /\(\d+.\d+%\) covered/`
```

The surrounding '/' is optional.

This commit:
* Turns `coverage_regex` into `coverage` entry in yml file
* Fixes smaller requests from code reviewers for the previous commit
* This commit is temporary (will be squashed afterwards)

This commit does not (further commits will do though):
* Add global `coverage` entry handling in yml file as suggested by Grzegorz
* Add specs
* Create changelog
* Create docs

* Instead of using the proposed `coverage` key, this expects `coverage_regex`