Enhance the sequence_diagram

[Plawn]

Enable a way to have two items on the same line, it could be an item, such as the input parts in the given image

or two arrows on the same line.

It could be a syntax such as :

example for two arrows on the same line

shape: sequence_diagram

item1 -> item2 : Link1 {
    # will be on the same line as the next item
    no-increment: true
}

item2 -> item3 : Link2 {
    # is normal
}

example for two items on the same line

shape: sequence_diagram

alice. "generate key" {
    no-increment: true
}

bob."generate key"

Thanks ! :)