[Multisite Content Copier] Copy items created or updated since a date or last time

To make it easier to only copy new items to sites, it'd be useful to be able to filter the list of items to copy based on a date.

It'd be even nicer if the plugin saved a date for when it was last run, so you could see content newer than that date and have the ability to select all.

Also, there is a typo on the plugin screen "click on Add itmes to list".