Add mail functionality

[zefir-git]

Mail allows you to send a private message to an offline player. They can read their mail once they log in.

When you log in, it tells you "You have 123 unread messages" if > 0.

Main commands: /mail or /email (not from essentials mail, but from E-Mail. lol)

/mail - view your inbox
/mail send <player> <message> - send mail
/mail sent - view sent mail and whether it was read
/mail view <id> - view a mail message
/mail markread - mark as read (similar cmd to mark unread)
/mail star <id> - star a mail message
/mail delete <id> - delete mail message
/mail archive - delete read emails (with confirmation. e.g. show confirmation message unless --confirm is added to the command)

save mail in sqlite