Added a option to manually set when the next Auto Clean-up will trigger.

[stanislavgeorgiev]

By default the Auto Clean-up will take the current time and schedule it for one week (or what the setting says) from now. I find that the optimize process is better to be run overnight when there's less if any people visiting my website. The manual setting is now allowing me to do so without the need of an additional plugin to modify my cron job run times.