Specify animation duration on individual links

Adds support for data-menu-duration attribute

@@ -15,6 +15,7 @@
Add MENU_DURATION_ATTR variable

@@ -132,10 +133,18 @@
Check if MENU_DURATION_ATTR atrribute is set on link.
If so, set animationDuration to attribute value, else use _duration() function

Proposed Documentation:

Duration
-----

To specify a set duration for the animation on a specific link, use the `data-menu-duration` attribute. This allows for fine-grain control between presentation slides, for example, where one slide may have more animation than others.


```html
<section id="kittens" data-menu-duration="2000">
	<h1>If it fits, I sits</h1>
	<p>Some text about felines (the internet loves felines).</p>
</section>
```

In this example, the transition to the #kittens anchor will take 2 seconds, over-riding the default.