Add a way to disable :Save() on player leave for specific stores

[Kampfkarren]

Use case: You want to teleport people to a new reserved server. You want to make sure you can get their data in the other server, so you :Save() before teleporting.

When they teleport away, they leave and their data is redundantly saved.

Alternatively, this could also be solved by correctly not resaving data if it hasn't been changed.

[Kampfkarren]

Made redundant with #48. If anyone has a good use case for this, I can reopen it.