Add option to clean old player's inventories in configuration, which will delete all multiverse-inventories data older than x days, as I've been using the plugin since 2015, and since then over 250.000 inventories have been stored, and I've noticed that only 7000 are those which were last-edited in 2017 (so those being the only active ones).

This way every mv-inventories inventory that was edited more than x days from now would be deleted and the inventory database would be much cleaner and easier to manage.

Example config:
// options: auto, manual (with command)
database_cleanupmode: auto
database_cleanupdays: 200
// options: load, unload
database_cleanupon: unload

Duplicate of #171.