Document _onWriteReport config option

Currently the only mention of the `_onWriteReport` config option is in the [source code](https://github.com/karma-runner/karma-coverage/blob/master/lib/reporter.js#L62). This is an extremely valuable option (in my case, which is get code coverage reports for HTML files which are compiled to JS first).

The suggest change is to update the `readme.md` file with an example of how you can specify a function for `_onWriteReport` and the arguments that are passed to it.

While making that change, would it be possible to fix #240 at the same time? If not, that's ok.

Start with lib/reporter.js at the documented _onWriteReport option to verify the function arguments and behavior. Update readme.md with an example showing how to configure the function and the arguments it receives; treat the requested #240 fix as optional unless its scope is clear.

Written by the indexing model from the issue text.