Add preventDefault() to avoid page scroll while panning

[gofr]

Enabling dragging of a map changes the touch-action CSS property
value to prevent the page from scrolling while you are dragging.

However, when dragging is enabled dynamically inside a touch event
this CSS change doesn't affect the current gesture.

Add e.preventDefault() in the event handler to prevent the page
from scrolling while you're performing a two-finger drag action
on the map.

This addresses issue #65

[jcroucher]

Can we get this merged

[elmarquis]

Apologies. I've not been getting a lot of time to look after this plugin. But this fix appears simple and effective. Thank you for the investigation and resolution.