Add support for different handling of YES vs NO answer

I used this plugin in a work project, but needed to perform a special action if the user pressed the "cancel" button, but there was no way to intercept this using this plugin, so I added this functionality.

The callback can now read from the argument ```confirmed```, which button the user pressed, and decide which action(s) to take.

example:
```javascript
$('#myButton').confirmOn('click', function(e, confirmed){
    if(confirmed) deleteSomethingImportant();
    else {
        //If we need to, we can do some actions here if the user cancelled the confirmation
    }
})
```