Rendering just vertices and edges

[edemaine]

We should support rendering of vertices and edges that do not appear in any faces. Many applications for this:

• Debugging. Maybe you've just defined your vertex set so far, and want to make sure you got it right.
• Tutorial. I'm working on a tutorial document where it would similarly be nice to be able to show "what we've built so far" as we go step-by-step through defining FOLD attributes.
• Linkages. One 6.849 group is interested in rendering linkages using FOLD.
• Crease patterns. A common use case, I think, is that software generates vertices and edges defining a crease pattern, and then take it into the FOLD web software to add faces automatically. Before clicking that "add faces" button it'd be nice to see the skeleton of the crease pattern.

Jason, is this hard to add? I don't think we don't need to worry about edgeOrders; we just need to do the "obvious" partial ordering between vertices, edges, and faces. And this is probably only necessary for vertices and edges that aren't a part of any face.