Add --subtract-fee flag to include the gas fee in the amount

[matevz]

Currently, there is no simple way to empty the account, because you will always need to leave some tokens to pay for the gas fee. Add --subtract-fee flag to transfer, withdraw, deposit commands which subtracts the required gas fee from the entered amount. By using this flag, you can check the account balance and then copy&paste it into the transfer command to empty it.

An alternative flag would be --all, but then the amount parameter would become optional which complicates things. Also, it allows you to completely empty the account and not leave, for example, some predetermined number of tokens in there.

The flag was inspired by the subtractfeefromamount argument used in bitcoin rpc: https://developer.bitcoin.org/reference/rpc/sendtoaddress.html