Add code coverage support

[arunoda]

It's better if we support code coverage support as well.

[arunoda]

@mnmtanish I hope you did some stuffs on this field right?

[thani-sh]

I tried isparta earlier but couldn't get it to work well in some cases. I read about this babel plugin just recently. If the babel-plugin works well we can use it or we'll just use isparta.

[arunoda]

I saw some uses istanbul ?

[xcv58]

Is ava an option?

[arunoda]

@xcv58 This is about code coverage. But unfortuantely, we won't support ava.
Parallel testing in something we tested with React testing specially since we use JSDOM.

[xcv58]

@arunoda sorry about this. I actually mean nyc for test coverage.