Add ol.geom.flat.intersectsextent

Includes functions for testing if an extent and a geometry intersect.
Add ol.geom.Geometry#intersectsExtent (abstract method)
Add ol.geom.Point#intersectsExtent
Add ol.geom.MultiPoint#intersectsExtent
Add ol.geom.Polygon#intersectsExtent
Add ol.geom.MultiPolygon#intersectsExtent
Add ol.geom.LineString#intersectsExtent
Add ol.geom.MultiLineString#intersectsExtent
Add ol.geom.GeometryCollection#intersectsExtent
Add ol.source.Vector#forEachFeatureIntersectingExtent
Add a box feature selection example