Simple boolean option on canvas to allow scrolling on touchscreen devices

I ran into a use case where I wanted the canvas to fill the entire browser window horizontally, as well as extend well below the available browser space vertically.  On touch devices this became an issue, because the fabric was catching the mouse/touch move events and preventing default behavior.

I can see `preventDefault()` being useful in certain situations, such as applications where the user is drawing on the screen or moving items around on the canvas.  However, in situations like the one I initially described, where the canvas is intentionally consuming all of the available screen real estate and more, scrolling could be desired.

I have added a check for `canvas.allowTouchScrolling = true;`, which allows the developer to make the decision as to whether they do or do not want to allow scrolling to occur on touch screen devices.  As it is not set by default, the default behavior of fabric will remain unchanged.