writeReport dest argument doesn't support function

[njs50]

in libr/writeReport.js it says:

     * @param  {string|function} dest          The filename or outputting
     *                                         function to use for generating
     *                                         the report

however when passing a function there is an error in the reporter that it expects a string.

it would be nice if this worked as per the istanbul coverage config (i.e the function is passed the browser name and returns the destination).