Zoom always goes to the center of the plot, not to where the mouse is

[chrisbolin]

(same for VictoryZoom and VictoryZoomContainer)

On many applications with zooming (e.g. Preview or Google Maps), zooming centers on the current mouse position. In Victory, however, zooming always goes to the center of the current domain. This makes it difficult to zoom to a particular part of a large data-set.