Loop support

[julianstanley]

Some of my graphs have loops.

With this tool, I cannot make a loop visually. It does support loops in an input json file, but the visualization is just a single arrow pointing at the node:

It would be great if I could add loops visually and have a nicer-looking representation, like this:

I feel like it should be pretty straightforward to do that, so I'll go ahead and give it a shot.