Drop grunt.util._

`grunt.util._` is [deprecated](http://gruntjs.com/api/grunt.util#grunt.util._).
So I replaced [`grunt.util._.forEach`](http://lodash.com/docs#forEach) with [`for ... in`](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Statements/for...in), and [`grunt.util._.isFunction`](http://lodash.com/docs#isFunction) with [`typeof`](https://developer.mozilla.org/docs/Web/JavaScript/Reference/Operators/typeof).

Another solution to be considered is requiring [`lodash`](https://npmjs.org/package/lodash) module and simply replacing `grunt.util._` with it. However, I think loading Lodash causes a lot of overhead, and I prefer that this plugin is independent from Lodash as much as possible.