1) I added a command queue which is helpful to me (at least so far) and
   would also allow syntax like 's;s' (step; step) in conjunction with precmd
2) doc_leader allows the derived class to print a message before the help
   output.  Defaults to current practise of a blank line
3) nohelp allows one to override the 'No help on' message.  I need
    'Undefined command: "%s".  Try "help".'
4) Pass line to self.precmd to allow one to do some parsing: change first
   word to lower case, strip out a leading number, whatever.
5) Pass the result of onecmd and the input line to postcmd.  This allows
   one to ponder the stop result before it is effective.
6) emptyline() requires a   if self.lastcmd:  conditional because if the
   first command is null (<cr>), you get an infinite recursion with the
   code as it stands.