Add docs for default js settings

I believe it's the intention that someone can override the default settings on Javascript controls by using the following code:

```js
$.fn.modal.Constructor.DEFAULTS.keyboard = true;
```

However, this doesn't seem to be documented anywhere.  This change just updates the [JavaScript page's programmaticAPI section](http://getbootstrap.com/javascript/#js-programmatic-api).
Oh, didn't notice that. Never mind then.