Document _onWriteReport config option

[uglow]

Currently the only mention of the _onWriteReport config option is in the source code. 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.