Hinting command when user types an invalid command

Maybe we can add an additional implementation to display similiar command when user types an invalid command
Example:

```
vorpal
.command('foo', 'Outputs "foo".')
.action(function(args, callback()) {
this.log('foo');
callback();
});

vorpal
.catch('[words...]', 'Catches incorrect commands')
.action(function (args, callback()) {
this.log(args.words.join(' ') + ' is not a valid command.');
// new function
// something like vorpal.similiar(args.words[0])
// to display similiar commands
callback();
});
```

When user type `node command.js foozz`, it will output:

```
foozz is not a valid command. Maybe you mean:
foo
```