Allow arbitrary RDBs

[strichter]

Describe the solution you'd like
Make CoverageData configurable through plugins. This way I can register my own CoverageData object with that can connect to other databases, such as MySQL or PostGreSQL.

This will allow me to merge my parallel run data into a central database after the measurement collection is done. No more managing of many files that need to be collected and combined.

Describe alternatives you've considered
I can monkey patch the library enough to make this work. :-)

I am willing to provide a PR for this feature.