Support for coffee files coverage

The code was taken from this task: https://github.com/karma-runner/karma/issues/622

Currently coverage plugin is broken for coffee script sources.
Even when coffeescript preprocessor is set before the coverage preprocessor, plugin fails on report generation because it tries to apply the report from the instrumented js file that was converted by coffee preprocessor to the coffee sources.

The solution from the link solves this problem.
Unit tests attached.