Coverage configuration with fast run and debug

Related discussion https://github.com/karma-runner/karma/issues/630

As expected coverage slows down test run and makes debugging impossible.
The workaround is to maintain two configs - the first one with coverage configured, and the second without coverage.
To improve the situation, there is a feature that allows external integrations (e.g. IDE) to turn off `coverage` preprocessor by removing `coverage` reporter: https://github.com/karma-runner/karma-coverage/blob/v0.5.1/lib/preprocessor.js#L54
This feature has been using by IntelliJ integration until a configuration that need `coverage` preprocessor for test run has emerged: https://github.com/karma-runner/karma-coverage/blob/v0.5.1/examples/coffee/karma.conf.coffee

Now IntelliJ uses a new trick to perform "Run", "Debug" and "Run with Coverage" actions with single karma.conf.js. However, it expects one thing from users - they should remove `coverage` reporter if `coverage` preprocessor is not needed for test run (still common use case, IMO):
- "Run" and "Debug" actions will work as `coverage` reporter is not set in config (done by users)
- "Run with Coverage" will work because `coverage` reporter is added on-the-fly by IntelliJ integration (https://github.com/karma-runner/karma-intellij/blob/2f63c75ea451d25fb17346abf8813e611d5e94f7/lib/intellijCoverageReporter.js#L19).

The downsides of the current approach:
- Undiscoverable for users
- Coverage won't be gathered if tests are run outside of IntelliJ, e.g. in Terminal

It'd be appreciated if it would be possible to improve the situation. A possible solution could be changing current coffeescript recommended configuration by making it possible to run tests without required coverage preprocessor. Also that would speed up tests.
Thanks.