Split number of files as argument

[proyb6]

It's prefer to set number of files to be split as argument Instead of a question.

-number 33

[jesseduffield]

I've added an -n flag for this, as well as a -t flag for setting the threshold. I'd prefer to use a package like flaggy to make it a little nicer (accepting different flag names for the same variable, not caring about where positional args are stored) but I don't want to have any dependencies (and I'm slightly lazy) so I'm just using -n for now (and keeping the prompt if you don't use the flag)