[vector-api] Add ol.source.Vector#addFeatures

This PR adds an `addFeatures` method to `ol.source.Vector`. This is for two reasons:

* So that we can take advantage of [rbush's bulk load](https://github.com/mourner/rbush#bulk-inserting-data) when it is ported
* So that only a single `change` event is fired when multiple features are added (note that `addfeature` events are still fired for every feature added)

Please review.