Add Screen Option for number of sales orders to show per page in Sales Log

This has annoyed me for the longest time.
You can use this patch to modify the per_page parameter in Screen Options on the Sales Log page.
I just added the third element to the array. It is really dumb WordPress code - you look for "your" option and return it instead of false. It is so that you can "override". If we recognize one of the options - we return the value. More like a failsafe.