Add code coverage

[housni]

See:

1. https://codecov.io/
2. https://medium.com/google-cloud/bash-code-coverage-using-codecov-with-kcov-travis-ci-303f3356981c

Implementation would, roughly, be like this:

$ mkdir -p coverage
$ docker run --security-opt seccomp=unconfined -v "${PWD}:/usr/lib/sheldon" -w "/usr/lib/sheldon" ragnaroek/kcov:v33 coverage/ ./test/test.sh