Add support for layout direction

[btiernay]

A small thing, but can really help with legibility and comprehension in some diagrams. I would generally think that vertical is a suitable default since scrolling is more natural.

[jfeliu007]

How would you suggest we do this? I find it a bit difficult to do if I have to add 'u' and 'd' to the lines in the diagrams since it is difficult to assess where to properly do it. Is there an option I can pass in the diagram itself that will make this change?