enable \n in vertex labels

[mjrmason]

Fantastic Package! Vertex labels with new lines are enabled in igraph plotting but not in this package. It would increase my use greatly if I could include new lines in my vertex labels like "Com 12\nMemmory T cells". Maybe a bug, maybe a enhancement... not sure.

[gvegayon]

Thank you, @mjrmason! That sounds like a great idea. I'll add it to the to-do list. That said, PRs are more than welcome :).