Issue title: Stop on point
 Issue body: I'm trying to stop an object on a point or percentage of the length of a path. I was wondering if there is a feedback loop for object's current position or simpler way to do this using your plugin. 

 Comments: 
 Comment 0: i don't think is easy.  u'll have to create a separate path :s as far as I know. unless jQuery has some new functionality to abort animations.

 Comment 1: Well, there is the step function of the method. I've been playing around with it. Although, I haven't exactly figure it out. I think it has the potential to function this way. 

However, when applying it to your plugin, the step function does not seem to return any values.

What do you mean create a separate path?

 Comment 2: Bah.. I didn't mean to hit Comment and Close...

 Comment 3: could u use stop (http://api.jquery.com/stop/) within the step function perhaps ? 

 Comment 4: Well, I haven't quite figured out how to call stop() from within the step function if you even can, but you can do some neat things with it. I have some code, but I don't know how to post it in there.

On the other hand, I can't get the step function to work while using your Path plugin. Perhaps I'm missing something?

 Comment 5: Ok, just figured out how to call stop() from step, but it doesn't work with your Paths plugin. The same data isn't being accessed when being pushed through Paths.

 Comment 6: Check it out.. 
Change
var css = fx.end.css(1 - fx.pos)

to 
var css = fx.end.css(fx.pos/2)

Could change the code to accept a param there maybe. 

 Comment 7: Alright, so I'm still hacking shelby53@example.org. I brought the divisor in as a param, not sure that's the way to go though. Playing around with some loops now to see if I can make the divisor work...

 Comment 8: try 

var css = fx.end.css(1 - fx.pos*param)  

where param is between 0 and 1

 Comment 9: I was able to get an animation to stop at an arbitrary point along the curve by using step and stop()

$("#mover2").animate({path : new $.path.bezier(bezier_params)},
    {step: function(now, fx) {
        if (fx.pos > .5) {$(this).stop(true, false)}; //when the position param of fx gets bigger than .5, stop the animation
        }
        }                                       

{'source': 'GithubIssues', 'repo_name_with_owner': 'weepy/jquery.path', 'issue_id': 658530, 'repo_stars': 412.0}