Implement event delegation

Implement [event delegation](https://developer.mozilla.org/en-US/docs/Web/API/Event/target).

**Option 1**

``` js
hoverintent(ele [, selector ], handlerIn, handlerOut);
```

**Option 2**

``` js
hoverintent(ele, handlerIn, handlerOut).options({
    on: 'selector'
});
```
