feat: new `subdir` option

We discussed in this [issue] about the problem of not being able to choose the
complete output directory.

This commit addresses this issue and add the new `subdir` option which allows to
specify the full output directory of each coverage report.

[issue]: https://github.com/karma-runner/karma-coverage/pull/62
Merge pull request #90 from karma-runner/configurable-subdir

feat: new `subdir` option