Faster file writer

This is a very trivial change that however greatly improves the performances of the reporter.

I have a huge repository, ~500 files and ~50k LOC, timing the grunt build (running only one test)

Original code, istanbul 0.3.0
```
Done, without errors.
Task 'istanbul:instrument' took 9 seconds
Task 'testem:coverage' took 15 seconds
Task 'istanbul:report' took 3 minutes 31 seconds
All tasks took 3 minutes 57 seconds
```

After this fix
```
Done, without errors.
Task 'istanbul:instrument' took 9 seconds
Task 'testem:coverage' took 15 seconds
Task 'istanbul:report' took 8 seconds
All tasks took 34 seconds
```