account: add option to rename account

[iBobik]

litcli accounts update can change only balance and expiration, but also label change is useful for good account management. Also help text should be changed to clarify --label is not for changing but only for identifying the account.

Current:

NAME:
   litcli accounts update - Update an existing off-chain account.

USAGE:
   litcli accounts update [command options] [id | label] new_balance [new_expiration_date] [--save_to=]

DESCRIPTION:
   Updates an existing off-chain account and sets either a new balance or new expiration date or both.

OPTIONS:
   --id value                   The ID of the account to update.
   --label value                (optional) The unique label of the account.
   --new_balance value          The new balance of the account; -1 means do not update the balance. (default: -1)
   --new_expiration_date value  The new expiration date of the account expressed in seconds since the unix epoch; -1 means do not update the expiration date; 0 means it does not expire. (default: -1)