implement ACCT command

Currently superfluous, responds with ```202``` to alert user to proceed anyway. Stubbed out for future extension.

Relevant test coverage included.

From RFC959:

         ACCOUNT (ACCT)

            The argument field is a Telnet string identifying the user's
            account.  The command is not necessarily related to the USER
            command, as some sites may require an account for login and
            others only for specific access, such as storing files.  In
            the latter case the command may arrive at any time.

            There are reply codes to differentiate these cases for the
            automation: when account information is required for login,
            the response to a successful PASSword command is reply code
            332.  On the other hand, if account information is NOT
            required for login, the reply to a successful PASSword
            command is 230; and if the account information is needed for
            a command issued later in the dialogue, the server should
            return a 332 or 532 reply depending on whether it stores
            (pending receipt of the ACCounT command) or discards the
            command, respectively.