Handle multi-graph edge rendering

[danielabar]

It's possible to have multiple edges between the same pair of nodes.
Currently edges are rendered as straight lines, so multiples end up drawn on top of eachother.
These should be rendered as paths with arcs so they can be seen as separate.

See this SO thread if it helps.

[danielabar]

Notes from @matanster in gitter discussion:

The best solution is probably to: have edges be arcs rather than straight lines, and space out the edges in the case of let's call them "parallel edges" for now, in a way that the belly of the edge is fatter for each parallel edge, e.g. if there's 3 parallel edges, they will be layed out with increasing "belly width".