[Feature Request] Make multithreading optional

[McBaws]

If you have more than a handful of large files, running all of the xdelta processes in parallel can be very slow with default settings, even eating up all the memory and crippling the computer.

I don't want to have to worry about tuning my source window size settings, so please add a toggle for multithreading or make the number of processes that run in parallel customizable. Thanks!