Suggest hubot commands when not found

Currently, Hubot doesn’t output any hints if we type a subtle mis-spelling. 
This PR enables to suggest commands close to input through a levenshtein distance algorithm.

The function looks like this:

```sh
Hubot> hubot youtobe me penguin
Hubot> Could not find command 'hubot youtobe me penguin'.
Maybe you meant 'Hubot youtube me' or 'Hubot time' or 'Hubot the rules'.
Run `Hubot help` for more commands.
```

I hope this function could be useful.