Add label zOrder configuration

[lucagrammer]

Hi, thanks for this great and very useful library. I would like to suggest a very small improvement that would greatly improve the usability, which is often compromised when using paths. Indeed, paths are the last element rendered on the SVG and therefore they cover the labels associated with the nodes, making them unreadable.

My suggestion is to change the rendering order or provide a zOrder option in configs to change it. In fact there is no other way (css z-index doesn't work with SVGs) to alter this property.

Thank you very much!