Add accordion "duration" option

Many widgets which perform animation allow the duration option to be specified either at ''construction'' time or via the ui.foo.defaults table, however, accordion does not. It instead hard-codes durations in its ui.accordion.animations table in a way which can not be easily overridden. It would be useful if accordion, like other widgets, would allow animation speed to be configured by the client.

It is true that one can ''subclass'', for instance, the "slide" animation to change its duration:

$.extend($.ui.accordion.animations, {
  fastslide: function(options) {
    $.ui.accordion.animations.slide(options, { duration: 100 }); }
  });

However, this approach is neither obvious, documented, nor consistent with other widgets which allow duration to be specified directly.