Image visualization in nodes

[hackl]

Display (selected) nodes using small images instead of simple circles.

[M-Lampert]

Will be added with #295 and can be used as follows:

g = pp.Graph.from_edge_list([("a", "b")])
pp.plot(g, node_image={"a": "https://avatars.githubusercontent.com/u/52822508?s=48&v=4"})

For now only URLs can be used and no local images in VSCode since VSCode does not allow displaying local files jupyter notebooks