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
<a href="#target-element" data-menu-duration="2000">Link to target element</a>
```

[edit: updated markup example]

In this example, the transition to the #target-element anchor will take 2 seconds, over-riding the default.