Nwdiag : create a route command

Is your feature request related to a problem? Please describe. John, as a network administrator wants to be able to identify the flows between devices and also how the packets will flow across a routed network

This feature was integrated in a branch of the original nwdiag and we are interested in having it available in the nwdiag port of plantuml

Describe the solution you'd like

 route {
        node1 -> node2;
        node2 -> node3;
    }

https://user-images.githubusercontent.com/24653321/68541791-2cabb280-03e7-11ea-9f19-7e39fa484145.png

This will show a line between those nodes on the assigned directions

More details available in the branch of nwdiag where this feature is implemented

Describe alternatives you've considered Using original nwdiag with adapted branch

Additional context

https://github.com/blockdiag/nwdiag/pull/15

plantuml / plantuml

Nwdiag : create a route command #1750