Allow comments in .jshintrc with jshintcli.loadConfig()

jshint has exposed the loadConfig method since v2.1.5.

Related:
* https://github.com/gruntjs/grunt-contrib-jshint/issues/2
* https://github.com/gruntjs/grunt-contrib-jshint/issues/79
* https://github.com/gruntjs/grunt-contrib-jshint/pull/80#issuecomment-21017860
* https://github.com/jshint/jshint/issues/741