Emit browserifyInstanceCreated when creating parcelify

We are using cartero in our project and we came across with the following use case: I want to create a common parcel (js, css and img set bundle) that can be shared across some of the parcels (taking advantage of browser cache). 

To achieve that I created a new parcel called commonBundle, which will require the common modules. Now I need to (browserify.)exclude those files from all the parcels except commonBundle. With this change I can call cartero like this:

```
cartero.on('browserifyInstanceCreated', function(parcelName, browserifyInstance) {
    if (parcelName !== 'commonBundle') {
      browserifyInstance
        .exclude('jquery')
        .exclude('commonStuff');
    }
});
```