Add option to stop continuous update when zooming

From our conversation in #553:

When using the brush to zoom into data, it's really cool how the graph updates as you go along, but when you have a lot of data it's very laggy. It might be better if there was an option where you can start dragging the brush to zoom and the actual zoom action on the chart only takes place after you have stopped zooming, rather than zooming as you go along.