Custom cookie - Create RSS feed from data behind login

### What feature is it?
Enable custom cookie option, to create RSS feed from data that's behind login. Many websites use ReCapchta protection against bot logins, that's why I suppose the --custom-cookie option which can be used anywhere.

### What problem does this feature solve?
Get data from website that's behind login. For example a forum that requires login to view forum.

### Additional description
The program youtube-dl does have --custom-cookie option if you are interested. But for RSSHub this option needs to be easily available when you create a new route for a new website.