allow travellers to be square

[davewhiteland]

It might be handy to have non-round travellers (so they look distinct from vertices, especially when carrying payloads).

This is related to #27 but would be a little easier (so maybe do this one first?): we already have a type which is currently spot or sprite. Seems like that could be

• circle (synonym for spot), square, sprite

...and unlike vertices there's no complication about edges/arrowheads joining neatly.

I thought I had a specific use case here, animating a stack for a push-down automaton, and pushing round travellers (which is how I've done it in other graphs where I was putting the vertex on the stack) was going to be confusing because they would look like vertices moving... but now I don't think it's relevant because I'll probably implement them off-graph (using Pixi directly). It's almost as if having a separate stack (outside the graph and its nodes) is a whole new needed feature. Hmm.

square has the advantage of not needing any more specification config than we already have (because we could, um, repurpose radius... too dirty/lazy? Maybe height and width since we can anticipate rectangles (and PIXI supports rounded rect too).