support grunt's src and dest matching

Usage:
webp: {

	assets : {
		files: [{
			expand: true,
			cwd: '<%= yeoman.app %>/assets',
			src: '500x716/{,**/}*.png',
			dest: '<%= yeoman.app %>/assets/500x716-webp/'
		}]
	},
	options: {
		binpath: require('webp-bin').path
	}
},