Add events for when you remove or select an item

[ssetrom]

This PR adds two convenient events:

1. "itemRemoved": An event for when you remove an item. The detail of this event is the item removed.
2. "itemSelected": An event for when you select an item. The detail of this event is the item selected.

I also added the "itemCreated" event to the documentation.

[rob-balfre]

@ssetrom thanks for the PR. There is already on:select and on:clear so not sure these add anything new.

[ssetrom]

@rob-balfre Ah, I should probably have written more about how these events are different. So 'itemSelected' is only ever triggered when the user directly changes a value, and it only returns the one value that was selected/added. This is important because the 'select' event is always triggered when the data changes ( It can happen because of multiple reasons, for instance the value prop changing ).

Upon some investigation, I've found that actually the 'clear' event totally does what I want it to do! Sorry about that. I believe I added that event for an earlier version, and it was probably solving some issue that no longer exists :)

[rob-balfre]

@ssetrom I think I'd prefer to change the PR to update on:select to only fire when an item is selected instead of having another event.

[rob-balfre]

Events in v5 can achieve same result.

Thanks for the PR though and very sorry for slow reply.