Add support for more dynamic config files

[ssbarnea]

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.