onZoomStart is not called for pinch

[tomeronen]

if pinch is enabled, then zoom happens without the onZoomStart being called. I think this is a quick fix. Can i open a PR to add the onZoomStart function before the zoom call?

function handlePinch(chart, state, e) {
  if (state.scale) {
    const {center, pointers} = e;
    // Hammer reports the total scaling. We need the incremental amount
    const zoomPercent = 1 / state.scale * e.scale;
    const rect = e.target.getBoundingClientRect();
    const pinch = pinchAxes(pointers[0], pointers[1]);
    const mode = state.options.zoom.mode;
    const amount = {
      x: pinch.x && directionEnabled(mode, 'x', chart) ? zoomPercent : 1,
      y: pinch.y && directionEnabled(mode, 'y', chart) ? zoomPercent : 1,
      focalPoint: {
        x: center.x - rect.left,
        y: center.y - rect.top
      }
    };

    zoom(chart, amount);

    // Keep track of overall scale
    state.scale = e.scale;
  }
}