--file option for piping reporter output to a file

This adds an option to dump the reporter output (just the reporter output - browser `console.log` calls still go to standard out) to a file. This fixes #83 and alleviates issues like #51 , and probably helps a few other issues I've seen. This gaurentees that if you use the JSON reporter you get valid JSON even if there are rogue `console.log` calls or error output from mocha-phantomjs itself, like the newly added `onResourceError` output.

@metaskills Thoughts?