Expose types as options: only concatenate selective types

This will expose optional types for concatenation.
if  `types` are not specified, it will concatenate both `css` and `js` as before.

```js
gulp.task('html', function () {
    return gulp.src('app/*.html')
        .pipe($.useref.assets({searchPath: '{.tmp,app}', types: ['css']}))
        .pipe(gulpif('*.js', uglify()))
        .pipe(gulpif('*.css', minifyCss()))
        .pipe(useref.restore())
        .pipe(useref())
        .pipe(gulp.dest('dist'));
});
```

Above task only concatenate `css` files.