改善一些multi_download实现.

Signed-off-by: Jack <jack.wgm@gmail.com>
添加一些settings相关的定义.

Signed-off-by: Jack <jack.wgm@gmail.com>
规范书写, 注意大小写.

Signed-off-by: Jack <jack.wgm@gmail.com>
为http_options添加详细说明.

Signed-off-by: Jack <jack.wgm@gmail.com>