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).
Needs an `id`. And a corresponding entry needs to be added to the nav (which is in another file)