Emit start and end indexes on change, to allow database fetching

[Herteby]

I'm integrating this with Meteor to fetch items on-demand from the database.
When it emits an update, I update the skip and limit of the subscription.

I created a pull request here: #22

[Herteby]

I can now smoothly scroll through a 50,000 item list that is being sent from the server on demand based on scroll location! 😄 Just some minor issues left...

[Akryum]

https://github.com/Akryum/vue-virtual-scroller#update-event