Support 0% coverage of non-required files

Pull request for issue #32 .
That's just how it was originally: 

https://github.com/SBoudrias/gulp-istanbul/blob/d8531f7efd8f8102c946de709153f85586dee5b6/test/main.js#L67

It appears to be suppressing the nested mocha reporting. I can rework this section if you'd like.