Add ability to customize tree layout

[Misiu]

For small trees default layout engine works fine, like in this example, but for larger trees (especially with many child nodes) default layout takes so much place.

I'd like to be able to specify type of layout for child nodes, to get this behavior.

So instead of

I would get this layout

I found this question on StackOverflow, but it is from 3 years ago so maybe something new was added that would allow this layout.

yFiles Organization Chart is ideal example of layout I have in mind.

Main idea is to "pack" items together and display as much as possible without need to scroll horizontally.

[mbostock]

I’ve no plans to implement this, but you’re of course welcome to fork the existing hierarchy layouts or implement you own. Good luck!

[Misiu]

This was quite easy to build, however I'm thinking about rewriting this after #62, #63 and #64 are fixed.

Also please consider adding example of how to extend a layout.