Support for check-coverage with isparta

[xjamundx]

isparta which I seem to need to support my es6 stuff doesn't provide the check-coverage function that comes with istanbul and therefore when I add coverage numbers into the config grunt-mocha-istanbul will fail.

Would it be possible to add a separate option to configure the coverage comand or default to istanbul inside of the executeCheck function?

[pocesar]

I'll close this for now, if isparta ever implements this, should work out of the box if it follows the same command line