Run batch ffufs [Improvement]

Hello, @joohoi !

Thanks for your great tool!

Is it possible to implement a feature to scan thousands of urls in one ffuf instance?

Why: I want to scan servers gentle (1 thread, 2 seconds delay between requests). For now to scan 5000 urls I need to start 5000 ffuf instances.

I've done that successfully but there are some problems:
1. because each ffuf uses linux sockets networking becomes really slow, unstable and with huge delays and latency problems (I've tried hundreds of sysctl's parameters but I can't make it work for 1000+ instances)

2. each ffuf instance eats ram and when there are thousands of them they eat a lot. And each TCP socket too.

It would be really cool if there will be implemented mechanism to handle multiple urls!