Allow attaching articles to sockets#52

This is somewhat related to #28 and is arguably a better solution for a common use case: I would like to be able to attach an article to a socket, rather than positioning it on a bone. In theory this would allow it to scale and rotate appropriately, which the get-bone-position method cannot do.

This would of course necessitate the ability to access sockets from Lua in the same way we can access bones, which I’m not sure is currently possible either, as well to detach articles from sockets.