Add excludePrefix option / Remove deprecated utility

* I added the `excludePrefix` option. (resolved #34).
* I added a test and readme for the `excludePrefix` option.
* I replaced `grunt.util._` with [lodash-node](https://github.com/lodash/lodash-node) because [`grunt.util._` is deprecated](http://gruntjs.com/api/grunt.util#grunt.util._).