Allow subsets of the graph to be printed

[drahnr]

          For `Polkadot` the output of the graph is quite hard to reason about, so one improvement I was thinking about is to add some parameter such that only selected subsystems are present in the graph. WDYT ?

Originally posted by @sandreim in #43 (comment)

[drahnr]

I think that is generally a good idea, but how would we decided which subsystems to show? If we'd say we limit by n "hops" we would quickly render the entire thing, where as one "hop" would probably be insufficient.

We need to carve out a more precise purpose so we can assess the idea against requirements better.

[drahnr]

I think it might make more sense to denote colors (foreground, background) in the rust proc macro, possibly color coding groups. WDYT @sandreim @vstakhov?