add option to preventDefault on scroll #222

With #109 the ability to respond to scroll events was brought back. However, later on the preventDefault was removed from the event handling code. So using https://gist.github.com/tacaswell/3144287 to implement scroll to zoom results in both zooming in and the whole notebook scrolling:

Is there an easy way as a user to add back the ability to capture scroll? If not, could an option be added (default false for consistency) to capture scrolling events?

Relevant code is here:

I'm not sure where on the python side would be correct allow setting of an boolean for toggling capturing scroll as figure objects in other backends don't have this problem. Maybe there could be a toggle inside fig.canvas?