Add nyc and coveralls

[ralphtheninja]

I've noticed that there are multiple code paths that aren't exercised during tests and I think we should add some coverage tool to help us figure out what needs to be tested.

What tools are there out there that we can use?

See level-ws.

[vweevers]

nyc and/or coveralls?

[vweevers]

The setup in memdown works well. If we add airtap to levelup, we get code coverage for free.

[ralphtheninja]

@vweevers Could you clarify a bit? memdown uses istanbul for coverage.

[vweevers]

Oops, you're right. Nevermind me, I mixed up some things. Let's pretend I said:

The setup in memdown works well. If we add istanbul to levelup, we get code coverage for free. 😸

[vweevers]

But we should probably switch to nyc.

[ralphtheninja]

@vweevers What about adding nyc to airtap and use airtap in levelup so we do get coverage for free? 😉

[vweevers]

I like it better if those two are separated. If you want to combine code coverage of node tests and browser tests, nyc probably has to be separated (to encompass both suites).

[ralphtheninja]

@vweevers Maybe code coverage could be our next "major" thing to focus on, once we have sauce labs running where applicable?

[vweevers]

If that helps us on level-js, maybe. Short-term we can also collect code coverage manually, and fill the biggest gaps.