additional idea of creating .repeat for String

Should we be promoting the use of mutating the core JS objects? It should usually be avoided unless you know what your doing. Normally it would be fine but as an example in a cookbook it might not be the best thing to teach.

Also I kinda like this better:

```coffeescript
repeat = (n) -> [1..n].map(-> "foo").join("")
```

But that isn't ES3 compatible. Maybe:

```coffeescript
repeat = (n) -> (for [1..n] then "foo").join("")
```

That's ES3 compatible.