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).
That seems reasonable, but there are ton of headings that don't have an ID and sidebar listing.  I was thinking of this as a subsection of the programmatic API section.  That's why I went for an h4 instead of an h3.