Get original file size before optimize

If `cwd` and `dest` directory is same like this, original file size report optimized image size.

````
module.exports = function (grunt) {
  grunt.initConfig({
    image: {
      release: {
        files: [
          expand: true,
          cwd: 'build/img/',
          src: ['**/*.{png,jpg,gif}'],
          dest: 'build/img/'
        ]
      }
    }
  })
};
````