Implemented a new feature: pan/drag in boundaries

[grafzahl-io]

Hi,
I needed a additional feature very fast available on npm, so I forked this project and added it myself.
Thanks for your awesome work - by the way!

My feature will add an option to the plugin and this enables a limitation of dragging and panning. The user can only pan/drag in the viewport of the svg. It is no longer possible to drag the object outside the viewport where the object might get lost.

My fork can be found here https://github.com/grafzahl-io/snap.svg.zpd.

Maybe you want to integrate this feature in the project.

Best regards from Germany!