Help for individual commands not available

```
demo> help
Commands available:
 - help
        This help message
 - exit
        Quit the session
 - show_updt <unsigned int>
        Show next x updates
demo> help show_updt
wrong command: help show_updt
demo> help exit
wrong command: help exit
```

When there are lot of commands, help for individual commands can be very useful.