documenting `options.process` feature allows manipulaion of cache items

found in tasks/manifest.js - was a *huge* help for a hack of mine, where I was getting directories in my output. I was able to quickly fix with something like:
  options: {
    process: function(item) {
      return grunt.file.isDir(item) ? '' : item;
    }
    // .. snipped
  }