Added support for bank accounts

This allows people with the ChestShop.bank permission to put $BankName on th
e first line of a ChestShop sign to indicate that the shop should be linked to t
he given bank account rather than their own account. Admins are able to link to
any account, and players only to ones they are an owner of, or, if enabled, ones
they are a member of. Seeing as the concept of bank accounts is different for R
egister, I did not enable support for bank accounts with it, as there was no obv
ious way to make it work.