Allow user to enable -multi option in siegfried

[kieranjol]

I've been experimenting with improving performance with various tools, and I've noticed that sf uses very low resources as its default is one file at a time. I've used the -multi flag with various values and performance skyrockets, with hard disk bandwith being the bottleneck.
Allowing a user to specify a value (between 1 and 256) for -multi could lead to major performance boosts.
I could probably add this if you felt like it make sense?

[tw4l]

Hi @kieranjol, apologies for the delay in responding but yes, this sounds like a great improvement! Feel free to submit a PR if you'd like, or I can try to find some time in the near future to add this.