Add support for more dynamic config files

While #274 added support for config files these are currently 100% static and there is no way to use js based config files or at least to be able to expand environment variables inside the JSON file.

In my particular case I need to make the name of the final cobertura report file more dynamic as I am looping over a set of tests and I do not want them override each other.

Another issue related to supporting only json format is that it does not support comments.

Start by reviewing the config-file support introduced in #274 and trace how configuration reaches the Cobertura report filename. Compare the existing JSON parsing path with the requested JavaScript configuration, environment-variable expansion, and comment support. Done should include a clearly chosen supported approach and coverage for dynamic report names and the accepted configuration syntax.

Written by the indexing model from the issue text.