Add struct item_data, mob_db and mob_data for plugins

http://hercules.ws/board/topic/7161-plugin-missing-hook-to-struct-item-data-and-mob-data/
Remove plugin data in item_data destruction.
Move mob_db deletion into separate function.
Remove plugin data in mob_db destruction.
Remove plugin data in mob_data destruction.
Add function for reading custom itemdb fields.

This functions called after reading normal fields and after
adding item to itemdb.
Update HPM hooks.
Merge pull request #416 from 4144/plugext

Extend plugins support in item_data, mob_db, mob_data