added a if for the case when defaults don't exist

I was trying to run the example given on the README and got always a error because of the optional arguments:

```
/Users/david/Code/nsp/node_modules/celeri/lib/plugins/help.js:135
            if(ops.defaults[op]) {
                           ^
TypeError: Cannot read property '--age' of undefined
```

So I confined the for loop in a if statement to make sure that doesn't get executed if .defaults doesn't exist, so far is working right. I'm not sure thought if 'defaults' was actually a typo and should it by optional (because that makes it work too)