New command-on-exit/entry flag

**Is your feature request related to a problem? Please describe.**
I was thinking about a feature, that would execute command on player region entry/leave. e.g /tp .

**Describe the solution you'd like**
A flag that would allow such action to occur.

**Describe alternatives you've considered**
I've been using WGExtraFlags, but the plugin brings more issues than features. I also tried using command blocks, but that doesn't seem to be an option.