node_recommender: Implement graph visualization

[ajpwahqgbi]

It would be nice to generate a huge SVG from a dot graph for various things, in particular for the prospective peer exploration.

• newly low-fee reachable nodes and edges connecting (new or existing) low-fee reachable nodes to new low-fee reachable nodes should be highlighted in green
• nodes with low-fee routability improvements and edges between (new or existing) low-fee reachable nodes to nodes with low-fee routability improvements should be highlighted in purple
• some sort of visualization of min cuts; perhaps it would be too busy, though, to highlight and dash all the edges that are in the min cut for a newly or improved low-fee reachable node