pinchZoom: Add option to change the zoom factor

[cbn-falias]

First of all, thank you for this great plugin!

We are currently working on a project where the required zoom can increase very heavily.
By adjusting the zoomFactor the mouse wheel zoom works just fine.
Unfortunately there is no such thing when zooming with pinchZoom.

Would it be possible to add a new option like pinchZoomFactor or zoomFactorPinch?

[Fuzzyma]

when you pinchzoom, your 2 fingers always stay on the same spot on the canvas. that means you can only zoom as much as you can spread your fingers. A zoom factor doesnt really make sense.

If you want that anyway you are welcome to create a PR. The code should be easy enough to understand.
e.g. I would start by multiplying the zoomAmount in the pinchZoom function by 2 and see if it works as you would expect