Better rendering of dependency plots ("$diagram()")

[richfitz]

The current code is just a stub, really and won't work for real networks. For example here is an issue from a nontrivial project (dfalster/tree-p -- currently private)

and here's another from richfitz/modeladequacy:

I think the idea is sound, but dealing with networks that are a reasonable size in a moderately automatic way is going to be hard! Especially as the target names for filenames can be quite long.

[richfitz]

DiagrammeR looks promising, and generating a force directed d3 graph (suggested by Karl Broman) is another option that sidesteps layout.

[dfalster]

Shift from igraph to DiagrammeR in cd6036572ab25f36b3a44a9260f5d657d03b84d9

[richfitz]

Yeah, good call.