Add tags to diagrams to be able to later highlight parts of the graph

[colombod]

After the markdown is parsed and a graph is created is often interesting to be able to focus or highlight some parts. This would be useful with diagrams like sequence and flow where the markdown often is related to some process or code. Using the diagram to show flows or interactive sequences.

I would like to be able to render things like a call stack as sequence diagram and then as I walk the call stack visualise where that point is along the sequence.