Idea: use code coverage reports as a quality index

Not sure about the rating. Maybe something like > 80% → +3pts and > 90% → +5pts ?

It’s pretty easy to query coveralls, just give it a commit hash and extract the `covered_percent` field:

```
$ curl -s https://coveralls.io/builds/020a4aacb7df323947b3a092b63e7b416705bdd7.json | jq .covered_percent
94.8892674616695
```

It should be pretty easy to query codecov too, but something is broken: https://github.com/codecov/support/issues/307