Allow custom base directories for coverage reports

Sonarqube expects the coverage reports to have relative paths, so the absolute paths used in the lcov reporter doesn't work. Even if Sonarqube was okay with absolute paths, I'm running it in docker, so the path wouldn't match the coverage report anyway. I would suggest exposing a new configuration option for the reporter to allow you to specify your own base path. If it's not set, you could keep using absolute paths, so as to avoid breakage.

It looks like #106 may have solved it, but it was closed due to inactivity.