Use the algorithm JSHint uses for excludes instead of big-O(n^2) array e...

````javascript
//in tasks/jslint.js
excludedFiles = grunt.file.expand(excludedFiles);

  files = grunt.file
    .expand(files)
    .filter(function (file) {
      return excludedFiles.indexOf(file) === -1;
    });

````
This is a big-O of (n^2) algorithm for file exclusion and is taking 10 seconds on my relatively small project which excludes '**/node_modules/**'.  By contrast, grunt-contrib-jshint takes a handful of milliseconds to do the same thing.

Please accept the pull request.