ol-ext: Splitter interaction

ol.interaction.Splitter is an interaction that acts as a split feature agent while editing vector features (LineString).
The features are cloned. The interaction fires a beforesplit and aftersplit event on the source. You can listen to the event on the interaction itself or on the source of the splitted feature.
The sources provided to the interaction must have a spatial index to be splitable (configured with useSpatialIndex set to true).

TODO: Split with more than one feature (when moving nodes)
NB: If you look to an interaction that split a linestring just on clicking a point on the feature, look at ol.interaction.Split.