add option useGhostImage

[masbaehr]

Added an option to disable manual override of ghost image because this will cause a very ugly shape in almost 99% of the cases.

This would allow for an easy fix for those issues without breaking compatibility for existing deployments. https://github.com/dabeng/OrgChart/issues/212

Just pass 'useGhostImage': false

And it will give you a nice drag image if the browser supports it