Set offset value dynamically by function call.

I needed this to position the tooltip based on some svg rectangles with different sizes, similar to how the title can be set dynamically.

In this example, I initially bound some data to some svg rects.

```javascript
$('rect.salary').tipsy({
  gravity: 'se',
  offset: function() {
    var d = this.__data__;
    var o = y(d.y0)-y(d.y1);
    return -o/2;
  },
  title: function() {
    var d = this.__data__;
    return d.y1 - d.y0;
  }
});
```