Feature request: Support additional "arrowhead" types

[annennenne]

Thanks for this great package!

I am looking for a good plotting option for partial ancestral graphs (PAGs), which may have circles as "arrowheads" (example below).

As far as I can tell, this is not supported in ggraph, as arrowheads are specified by grid::arrow() which doesn't allow for using alternative arrowhead types. Would it be possible to add this feature?

As for alternatives, it's not possible in igraph either. It can be done inRgraphviz, but that requires Bioconductor installation and hence is not great for package dependencies (I want to include a plot method for PAGs in my package causalDisco) . So it would be very useful to have this available in ggraph!