[vector-api] Add ol.FeaturesOverlay#addFeature and #removeFeature

This PR adds `addFeature` and `removeFeature` convenience methods to `ol.FeaturesOverlay`. This is to consistent with `ol.source.Vector`, and other parts of the library (e.g. `ol.Map#addLayer`). This PR also changes the vector-layer example to demonstrate how to use an `ol.FeaturesOverlay` object for feature highlighting. Please review.