Feature request: add support for class "dagitty" to as_tbl_graph

[grasshoppermouse]

https://github.com/jtextor/dagitty

This package is widely used in analysis of graphical causal models (DAGs).

library(dagitty)
g <- randomDAG(10, 0.5)
class(g)
names(g) # nodes
edges(g)

[thomasp85]

I'll be happy to review a PR for this