Add support to override `siblingSeparation` for children of single node.

[tahirwaseer]

I've gone through the documentation and I was not able to find anything that could override the siblingSeparation for children of a specific node. I think it would be great if we can manually specify the children separation at node level. If there is already some way to do this please guide me and if possible add to documentation. For clarification: I am talking about x-axis space between child node of a parent. May be it will not be a feature required by everyone but with all the utilities and control that TreantJS provides I think this feature would add more value to it and will give more freedom to write even more customized solutions. Like we have childrenDropLevel and pseudo nodes to manipulate tree vertically.

[tahirwaseer]

Closing due to inactivity. But still that support be good.