Added option to show Istanbul text summary report on threshold pass

For visibility of your coverage, instead of just showing that your coverage is above thresholds I wanted to log out the Istanbul text summary report.

This will output something like the below image to the console, directly from Istanbul.

![istanbul-text-summary-report](https://cloud.githubusercontent.com/assets/175278/4214804/b025f076-38c7-11e4-8c7b-746715003ac3.png)

I have added tests for this option.