add support to connect nodes w/ each other

[eisber]

• also supports HTML nodes

[bigbug]

Thanks for the PR. It's a good idea to also integrate the drawing possibility. However, I think some more adjustments have to be made. So I would love to see some kind of synchronisation between the dot code and the graph. Otherwise the made changes will be lost on changing the code or reopening the file.

[eisber]

at least in my scenario I'm not sure how to accomplish the synchronization. it doesn't actually create an edge - it has an effect on the internal model and is probably reflected by a change of color in the table source.