using jquery object as target breaks jquery

Hello,
Your code should support target option as jquery object because in structured programming sometimes we dont have id`s but we do have reference ot jquery objects and your code stricts user to use either id or class on context-menu div. Restriction is never good.
```
var target = $('#context-menu');
$('.context').contextmenu({
  target:target , 
 })
```
It can be fixed by replacing 
```
this.$elements.attr('data-target',this.options.target)
```
with
```
this.$elements.data('target',this.options.target)
```

I pulled above changes and now works flawlessly.
Now also accepts jQueryObjects/References as targets in options along-with id or class selector