Use .jshintrc if it exists and no jshintrc option specified.

I was migrating some modules from using jshint CLI to grunt-contrib-jshint, and I found out that unlike jshint CLI, grunt-contrib-jshint ignores cwd/.jshintrc if it exists.

This PR allows grunt-contrib-jshint to use cwd/.jshintrc if the file exists and if there's no custom jshintrc path specified.
This way grunt-contrib-jshint's behaviour is closer to jshint CLI.