perhaps reuse `grid-gap` to specify sequence diagram vertical gaps

[alixander]

And maybe rename to gap-size

shape: sequence_diagram 
A -> B: Do something\nfun
B -> C: Pass the fun\nfurther
C -> D: Hey\nCheer up!
D -> C: Thanks,man { style.stroke-dash: 2 }
C -> B: Thank you { style.stroke-dash: 2 }
B -> A: Cool { style.stroke-dash: 2 }

[alixander]

also been a request for horizontal padding