Use both the '$$cov_*' and istanbul's default `__coverage__` var to see if istanbul is running

Currently it's only looking for the $$cov_ global variable to know if istanbul is running.  If you use istanbul programmatically, it doesn't instantiate $$cov_ but uses the default `__coverage__` var.  (see https://github.com/gotwarlost/istanbul/blob/master/lib/instrumenter.js#L340)  A great example of using istanbul programatically is https://www.npmjs.org/package/gulp-istanbul.