WORLDGUARD-2373 - Reported by Alastos

Add a flag that triggers a specified command when entering or leaving a region.

Example: I want to create a region where players are given creative mode and add the game-mode flag, however, I don't want them to be able to take anything with them when they leave the region.

/region flag run-cmd-exit /ci name

Something this powerful would obviously be limited to being used by admins/owners/OPs and would likely only be able to send commands that are able to be sent from the console.
Perhaps making it group specific, so the flag checks the group permissions to see if that group has permission to use that command. One example for that:

/region flag run-cmd-entry /msg name has just entered region id