Add code coverage reporting

[coryhouse]

I'm interested in including code coverage reporting within Slingshot. Isparta and nyc are two top candidates to consider.

[jeremyadavis]

I'll try to look into this. I'm about 85% done with my unit tests for my app build on slingshot, then I was gonna create a PR with my recommendations.

In my research, I've also seen a few mentions of Istanbul for coverage so I'll add that as a candidate to consider as well.

[coryhouse]

Excellent! Istanbul is expected to add ES6 support in its next major release, but until it does, it's not an option.

[nickytonline]

There is also remap-istanbul.

[dwmkerr]

Created PR #142 for this if you are interested guys 😄